significantly less than for CCP, which was significantly less than for venous outflow.

Transcription

1 Journal of Phy8iology (1988), 400, pp With 6 text-fitgures Printed in Great Britain VASCULAR CHANGES DURING PENILE ERECTION IN THE DOG BY C. J. CARATI, K. E. CREED* AND E. J. KEOGH From the Impotence Study Group, Reproductive Medicine Research Institute, Department of Clinical Biochemistry, Queen Elizabeth II Medical Centre, University of Western Australia and *School of Veterinary Studies, Murdoch University, Western Australia, Australia (Received 7 July 1987) SUMMARY 1. The vascular effects of pelvic nerve stimulation on the penis were studied in dogs anaesthetized with sodium pentobarbitone and halothane. Changes in pressure and blood flow were measured through scalp vein needles inserted into the erectile bodies. 2. The penis contains two types of erectile body, which responded independently during erection induced by pelvic nerve stimulation. Pressure in the corpus spongiosum increased immediately upon stimulation, but only reached one-third of the more delayed pressure response of the corpora cavernosa. 3. At rest, arterial inflow resistance into the corpora cavernosa was high, whereas venous outflow resistance was low. Pelvic nerve stimulation (10-50 V, Hz, 1 ms) caused an immediate increase in arterial flow, an increase in corpus cavernosal pressure (CCP), and a decrease in venous outflow. Saline infusion experiments showed there was active venous occlusion. Upon cessation of stimulation, these parameters returned to pre-stimulation levels. 4. The time taken to reach 50 % of maximum change in arterial inflow was significantly less than for CCP, which was significantly less than for venous outflow. Occlusion of the aorta 1 min after cessation of stimulation decreased the pressure in the arterial tree supplying the corpora cavernosa, but CCP remained elevated, indicating that both inflow and outflow resistances were high. Thus, inflow resistance had returned to its pre-stimulation state before outflow resistance. 5. Direct measurements of blood flow through the corpus cavernosum were made with a hydrogen probe. There was a transient increase in blood inflow as CCP increased during pelvic nerve stimulation. There was some blood flow while CCP was elevated, indicating that the venous occlusion was not complete. 6. Sympathetic chain stimulation caused an increase in arterial resistance, and a decrease in CCP and venous resistance. 7. Infusions of acetylcholine (330 jug min-') and vasoactive intestinal polypeptide (1-3-3,slg min-') decreased arterial resistance and increased CCP and venous resistance. 8. This study suggests that during pelvic nerve-induced erection, arterial flow into the corpus cavernosum increases, followed by an increase in CCP and an actively * From whom reprints should be requested.

2 76 C. J. CARATI, K. E. CREED AND E. J. KEOGH controlled decrease in venous outflow. All three parameters can be inhibited by sympathetic chain stimulation and mimicked by infusions of acetylcholine and vasoactive intestinal polypeptide. INTRODUCTION There is an immediate increase in blood flow into the penis during erection in the dog, followed by an increase in penile volume (Eckhard, 1863; Andersson, Bloom & Mellander, 1984) and elevation of corpus cavernosal pressure (Carati, Creed & Keogh, 1987). Whilst this clearly results from dilatation of the penile arteries, there are conflicting reports concerning venous occlusion during erection. Dorr & Brody (1967), Andersson et al. (1984) and others have reported a free flow of blood from the dorsal penile veins of anaesthetized dogs during erection. However, Lue, Takamura, Umraiya, Schmidt & Tanagho (1984) reported active venous occlusion in the same species. This issue is resolved by the recognition that the penis has two erectile systems: the high-pressure corpora cavernosa and the low-pressure corpus spongiosum and glans penis (Christensen, 1954: Purohit & Beckett, 1979). The canine corpus cavernosum is surrounded by a tough tunica albuginea and changes little in volume during erection, whereas the glans penis swells considerably (Christensen, 1954; Ninomiya, 1980). The observation of pressure differences between the corpora cavernosa and the corpus spongiosum during intercourse in a number of species led Purohit & Beckett (1979) to suggest that the corpus cavernosum was a 'closed system', whereas the corpus spongiosum was a 'one-way flow system', draining freely through the penile veins. However, the relationship between vascular and pressure changes during erection is unknown. Erection in the dog can be induced by electrical stimulation of the parasympathetic pelvic nerve (Eckhard, 1863; Langley & Anderson, 1895) and can be inhibited by activity of the sympathetic chain via sacral, but not hypogastric, fibres (Carati et al. 1987). Nothing is known of the effect of sympathetic activity on blood flow through the penis. The erectile tissue of the canine corpus cavernosum contains cholinergic and vasoactive intestinal polypeptidergic receptors (Carati, Goldie, Warton, Henry & Keogh, 1985). However, atropine, which blocks the actions of acetylcholine, had no effect on the cavernosal pressure changes produced by pelvic nerve stimulation in dogs (Carati et al. 1987), although it inhibited the increase in penile volume (Andersson et al. 1984). Studies of the role of vasoactive intestinal polypeptide (VIP) have produced conflicting results (see Carati et al. 1987). This study investigated the integrity of the corpus cavernosal and spongiosal systems in the dog, and the temporal relationship of the vascular and pressure phenomena associated with erection, as induced by pelvic nerve stimulation. The role of the sympathetic innervation, acetylcholine and VIP in modifying blood flow through the penis was also investigated. A preliminary account of this work has been published (Creed, Carati & Keogh, 1986). METHODS Twenty-six young adult male dogs (10-35 kg) of several breeds were anaesthetized with sodium pentobarbitone (30 mg kg body weight-', i.v.) and halothane (0-5-2 %, as required). Respiration

3 BLOOD FLOW DURING PENILE ERECTION was spontaneous during experimentation. Blood pressures were recorded on a Grass polygraph connected to Statham pressure transducers (P23). Systemic arterial pressure was monitored via a cannula in the carotid artery. The abdominal cavity was opened through a mid-line incision. The two sympathetic chains were identified at the 5th lumbar vertebra, tied, sectioned and the caudad end mounted on bipolar platinum electrodes. The right pelvic nerve was divided lateral to the rectum, and the distal end also mounted on electrodes. Supramaximal electrical stimulation (10-16 Hz) was delivered with trains of pulses of V and 1 ms pulse duration from a Grass stimulator (SD9). The pelvic nerve was stimulated for periods of 2 min followed by a 5-10 min rest. To measure local arterial pressure, a 0-63 mm o.d. cannula was inserted into the right dorsal artery of the penis and secured so that its tip was close to the arterial branches to the corpus cavernosum (artery of the penis or penile artery; Christensen, 1954). The pressure within the corpus cavernosum was measured through a 21 G scalp vein needle inserted into the corpus 3-4 cm proximal to the os penis. Pressure in the left corpus cavernosum and the corpus spongiosum was also measured through 21 G scalp vein needles. Arterial inflow was investigated by inserting two 19 G needles into the right corpus cavernosum, proximal to the 21 G needle, at the penoscrotal junction. These were joined with a Y-piece. The dogs were heparinized (400 U kg body weight-', David Bull Laboratories). Each drop of blood from the corpus cavernosum was recorded manually as it flowed from the Y-piece, at atmospheric pressure, into a container. Since changes in the volume of the canine corpus cavernosum are negligible (Ninomiya, 1980; our unpublished observations), this method gives a good indication of changes in arterial inflow. To investigate venous outflow, the carotid cannula was connected to the corpus cavernosum via a 3-0 mm o.d. extension tube attached to the Y-piece. The flow through this shunt was recorded with a photoelectric drop counter connected to the Grass polygraph. During most of these experiments, the normal arterial blood supply to the penis was blocked when the aorta was occluded by pulling up on an umbilical tape placed round the vessel just above the bifurcation. This produced a large drop in pressure in the penile artery. In some dogs, saline was infused into the corpus cavernosum through the two 19 G needles at a constant rate from a Harvard 975a infusion pump. In four dogs the rate of blood flow through the corpus cavernosum was measured with a hydrogen probe, as described by Miller & Ng (1981). This technique utilizes the dissociation of molecular hydrogen into hydrogen ions on the surface of an electrode which results in a measurable current, the magnitude of which is proportional to the concentration of hydrogen in the vicinity of the electrode. The dog was prepared as described above, with the pelvic nerve mounted for stimulation, and a 21 G needle inserted into the corpus cavernosum. In addition, a platinum electrode was inserted into the same corpus cavernosum, and a reference electrode into an adjacent muscle. The electrodes were attached to an amplifying unit and chart recorder. The dogs breathed a mixture of 14 % hydrogen in air, until the current between the electrodes was stable. Hydrogen administration was stopped and the current fell over the next 15 min. The hydrogen is removed from the tissue solely by blood perfusion, so the decay of current provides an estimate of blood flow. The half-time of the decay in current was measured and blood flow rate calculated in millilitres per minute per 100 grams by the equation 60 x 0-693/half-time. The rate was calculated before, during and after pelvic nerve stimulation. Drugs were injected or infused into the penile artery (via the catheter in the dorsal artery) or into the corpus cavernosum through a three-way stopcock in the carotid shunt. They were acetylcholine chloride (ACh, Sigma), and vasoactive intestinal polypeptide (VIP, Sigma, or as supplied by Professor Mutt, Karolinska Institute, Stockholm). All drugs were in heparinized saline. Statistical analyses were by Student's t test, with a significance level of Values are presented in the text as mean + standard error of the mean. 77 RESULTS Pressure responses in the penile bodies The mean systemic arterial pressure was mmhg. Resting penile artery pressure was % of mean systemic blood pressure and dropped to % on pelvic nerve stimulation at 10 Hz (see also Carati et al. 1987). The pressure responses of both corpora cavernosa to unilateral (right) pelvic nerve stimulation were similar,

4 78 C. J. CARATI, K. E. CREED AND E. J. KEOGH with pressures reaching penile artery pressure (PAP) from a resting level of mmhg (Fig. 1). This occurred after a latency of s. At the end of stimulation there was a parallel increase in PAP and corpus cavernosal pressure (CCP). CCP was maintained, occasionally for up to 6 min, before returning to prestimulation levels. The corpus spongiosum pressure was lower than that recorded in the corpus cavernosum (Fig. 1). On stimulation it rose immediately from 5-10 mmhg at rest to BP 160 (mmhg) R ight I (mmhg) 0 Left \ CCP I (mmhg) J CSp 30. (mmhg) J PAP 140 (mmhg) 100 PNS 2 min Saline infusion 30 ml min-' Fig. 1. Systemic blood pressure (BP), right and left corpus cavernosum pressure (CCP), corpus spongiosum pressure (CSP) and penile artery pressure (PAP) (mmhg) during 2 min of pelvic nerve stimulation (PNS, 10 Hz), or saline infusion into the right corpus cavernosum. The right and left CCP and the CSP response were independent of each other. CSP increased more rapidly than, but was only one-third of, CCP. reach mmhg (20-44 % of mean systemic arterial pressure) after 5-10 s. The bulbus glandis and pars longus glandis of the penis were swollen and the entire penis was turgid during stimulation. To observe whether the pressure increases were independent in the two corpora cavernosa and the corpus spongiosum, the pressure in one corpus cavernosum was elevated for 30 s by infusion of saline at up to 50 ml min-'. Pressure in either corpus cavernosum could be raised to 500 mmhg without affecting pressure in the other corpus cavernosum, the corpus spongiosum, or the penile artery (n = 3, Fig. 1). At the end of the infusion, the pressure immediately returned to the basal level. There was also no transfer of pressure between the corpora during pelvic nerve stimulation, since rapid injection of ml of saline in the right corpus cavernosum caused rapid and prolonged elevation in pressure of up to 500 mmhg, which was not recorded in the left corpus cavernosum or the corpus spongiosum (n = 3). Arterial inflow during pelvic nerve stimulation To assess the dependence of corpus cavernosal pressure (CCP) on arterial inflow, the aorta was occluded before, during and after pelvic nerve stimulation (n = 5).

5 BLOOD FLOW DURING PENILE ERECTION Occlusion in the absence of stimulation had no effect on CCP, but prevented its rise upon subsequent stimulation. When performed during pelvic nerve stimulation, CCP fell in parallel with PAP (Fig. 2). However, there was no drop in CCP if the aorta was occluded 1 min after pelvic nerve stimulation was stopped, and CCP remained elevated. In further experiments, the two 19 G scalp needles in the corpus cavernosum were opened to the atmosphere. There was no change in resting CCP, but during pelvic 79 BP.... (mmhg) 1504 CCP (mmhg) I 10 1J PAP (mmhg) 01 r PNS 2 min Aorta occluded Fig. 2. Effect of occluding the aorta during and after pelvic nerve stimulation (PNS, 10 Hz). Occlusion during stimulation decreased corpus cavernosal pressure (CCP) and penile artery pressure (PAP), whereas CCP remained elevated during occlusion 1 min after stimulation. Fluctuations due to respiration are superimposed on the blood pressure (BP) traces. nerve stimulation CCP was 45+5 % lower when the needles were open than when they were closed (n = 9, P < 0-001, paired t test, Fig. 3). We were unable to completely prevent a change in CCP during pelvic nerve stimulation, presumably due to resistance to blood flow through the two 19 G needles. In preliminary experiments, a 14 G fistula or three 19 G needles were no better and more invasive than using two 19 G needles. In the absence of stimulation there was very little or no flow through the 19 G needles ( drops min-', n = 10). Pelvic nerve stimulation resulted in an immediate increase in blood flow into the corpora cavernosa (Fig. 3). Within the first s of stimulation there was a mean increase in flow of 29-fold, to a maximum which was maintained for the duration of stimulation ( drops min-', n = 10). This increase in arterial inflow occurred more rapidly than the increase in CCP recorded in control pelvic nerve stimulations. The mean time for flow to reach 50 % of its maximum was significantly shorter (P < 0-005) than the time taken to reach 50% of maximum CCP (Table 1). Upon stopping stimulation, the increased flow from the 19 G needles was maintained for up to 1 min, after which it returned to its pre-stimulation rate. The time course of this recovery was not significantly different from the recovery of CCP (paired t test, P > 041, Table 1).

6 80 C. J. CARATI, K. E. CREED AND E. J. KEOGH Venous outflow during pelvic nerve stimulation Carotid artery-cavernosal shunt. When the corpus cavernosum was connected to the carotid artery, CCP rose from to mmhg (n = 8, Fig. 3). This increase was presumably due to a lower resistance through the carotid shunt compared to the normal arterial supply to the corpus cavernosum. When the aorta was subsequently occluded, there was often a slight increase in flow, which quickly stabilized as blood 100 (mmhg) ] 0 PAP 100 (mmhg) 50 PNS CCP 50] (mmhg) 0 Arterial drops l11 1I I PAP (mmhg) 100 l 50 J PNS 100 CCP (mmnhg) 0] Venous drops )_t l +Il & t. I PAP 100 (mmhg) 50 PNS Aorta occluded 1 min Fig. 3. Corpus cavernosal pressure (CCP) and penile artery pressure (PAP) during pelvic nerve stimulation (PNS, 10 Hz). In the middle record, arterial inflow was estimated with two 19 G needles draining the corpus cavernosum (Arterial drops), and in the bottom record venous outflow was estimated from flow through a carotid shunt connected to the 19 G needles at the arrow (Venous drops). Pelvic nerve stimulation increased arterial inflow, increased CCP and decreased venous outflow. from the carotid shunt perfused the corpus cavernosum ( drops min-'). Maximum CCP during pelvic nerve stimulation in these experiments were not significantly different from those recorded during control pelvic nerve stimulation (paired t test, P > 005), reaching % of mean systemic arterial pressure. Pelvic nerve stimulation caused a slight decrease in CCP and a slight increase in shunt flow rate over the first 15 s. This was followed by an increase in CCP, and usually an increase in outflow resistance, as evidenced by a decrease in flow through the carotid shunt (Fig. 3). The flow decreased by 46-fold from the pre-stimulation rate in nine of eleven dogs (to a minimum of drops min-'). The mean time course of this decrease in flow was significantly slower than for changes in CCP and

7 BLOOD FLOW DURING PENILE ERECTION arterial inflow (paired t tests, P < 0 001, Table 1). In seven of the nine animals, flow rate did not reach its minimum within the 2 min period of stimulation and it continued to decrease after stimulation was stopped. In two of the eleven dogs tested there was no decrease in blood flow through the shunt during pelvic nerve stimulation, and there was little or no increase in CCP during the shunt experiments. These animals had normal venous drop rates in the absence of stimulation and normal responses to control pelvic nerve stimulation. When the experiments were repeated with the aorta open, so that the penile artery was at normal pressure, CCP rose and flow through the shunt slowed. TABLE 1. Time (s) taken to reach 50% of maximum change in corpus cavernosal pressure (CCP), arterial inflow and venous outflow during and after pelvic nerve stimulation CCP Arterial inflow Venous outflow Changes during pelvic nerve stimulation Recovery after pelvic nerve stimulation Data are presented as means + standard errors of eight to nine animals. The time taken to reach 50 % of maximum change in arterial inflow after beginning stimulation was significantly less than for CCP, which was significantly less than for venous outflow (paired t tests, P < 0005). The changes were not significantly different during recovery. 81 The recovery of blood flow rate through the carotid shunt was more variable than the recovery of CCP and arterial inflow after pelvic nerve stimulation was stopped (Table 1). In five of nine dogs the flow had returned to pre-stimulation values with a time course similar to that for the recovery of CCP; in three, the flow was quicker to return to pre-stimulation values than CCP, whereas it was slower in the remaining dog. Constant-volume infusion. Responses to saline infusion at a constant flow rate were variable within and between animals. Infusions of saline in excess of 2-4 ml min-1 were required to elevate CCP in the absence of stimulation (n = 4). This implies that blood outflow resistance allowed extra flow of up to 2-4 ml min-' from the flaccid corpus cavernosum without affecting CCP. Infusions of up to 12 ml min-1 caused rate-dependent elevations of CCP (up to 80 mmhg) which were transient, falling to approximately half their maximum value within 1 min and plateauing at this level (n = 5). Infusion rates over 12 ml min-1 resulted in sustained elevations of CCP equal to systemic pressure (n = 8, Fig. 1). When saline was infused at rates of 021, 0-42 or 0-82 ml min-1, and the pelvic nerve stimulated, CCP initially increased as in the absence of infusion, but instead of reaching a plateau at close to systemic pressure, CCP continued to rise (n = 4, Fig. 4). The level reached varied widely between animals, from slightly above control CCP values to greater than 1000 mmhg in one case. This could only occur if blood outflow resistance was increased during pelvic nerve stimulation. The rate of rise and level of CCP increased with the rate of infusion. There was no effect on pressure in the penile artery during elevations in CCP of up to 1000 mmhg.

8 82 C. J. CARATI, K. E. CREED AND E. J. KEOGH Direct measurements of blood flow through the corpus cavernosum To correlate arterial and venous changes in terms of blood flow an estimate of flow was obtained with a hydrogen probe placed in the corpus cavernosum of 'hydrogenloaded' dogs. The blood flow rate in the flaccid state (13+1 ml min-' 100 g-1 of tissue, n = 4 dogs) was low compared to flow through the adjacent thigh muscle ( ml min g-1, n = 3, Table 2). Immediately after pelvic nerve stimulation, CCP (mmhg) PAP (mmhg) (mmhg) 0 PAP (mmhg) n J IAAA-. CCP/ (rmmhg) 0 \ PNS 2 min Saline infusion 0-21 ml min-' PAP 120 (mmhg) 0 Saline infusion 0-42 ml min' Fig. 4. Effect of saline infusion during pelvic nerve stimulation (PNS, 10 Hz) on corpus cavernosal pressure (CCP) and penile artery pressure (PAP). Low infusion rates caused large increases in CCP during PNS, which were not reflected in PAP. and during the rise in CCP, there was a significant 6-5-fold increase in flow. When CCP plateaued, however, blood flow decreased to a level similar to that observed in the flaccid penis. When pelvic nerve stimulation was stopped, the flow increased as CCP returned to pre-stimulation values. Effect of sympathetic chain stimulation on penile blood flow Stimulation of the sympathetic chain inhibited the increase in CCP produced in response to pelvic nerve stimulation (Carati et al. 1987). The effects of the sympathetic chain on flow were therefore investigated. Sympathetic chain stimulation alone (10 Hz, V) affected both arterial inflow and venous outflow and caused a slight increase in CCP (Fig. 5). In two of three dogs arterial inflow was slightly increased and in three of four dogs venous outflow (i.e. carotid shunt flow rate) was slightly decreased. Sympathetic chain stimulation delayed or prevented the rise in CCP due to subsequent simultaneous pelvic nerve stimulation, and increased the speed of its recovery. In arterial inflow experiments (n = 3), sympathetic chain stimulation prevented subsequent pelvic nerve stimulation from increasing flow from the 19 G

9 BLOOD FLOW DURING PENILE ERECTION 83 needles (Fig. 5), suggesting that arterial inflow and the consequent rise in CCP was prevented. Pelvic nerve stimulation resulted in CCP falling from the elevated levels caused by prior activation of the sympathetic chain, and there was an increase in flow through the carotid shunt, suggesting an increase in venous outflow (n = 4). When the sympathetic chain was stimulated in the middle of pelvic nerve TABLE 2. Blood flow rate through the corpus cavernosum as measured by a hydrogen probe Blood flow rate (ml min-' 100 g-') Pre-stimulation, CCP low and steady Pelvic nerve stimulation, CCP increasing * Pelvic nerve stimulation, CCP plateaued Post-stimulation P6t Flow rate was calculated from the rate of decay of hydrogen concentration in the corpus cavernosum of hydrogen-loaded animals. Data are presented as means+standard error of four animals. There was a significant increase in flow as CCP was both increasing (* paired t test, P < 0001) and decreasing (t paired t test, P < 005). Arterial CCP 100 (mmhg) 0 Drops I ItI Ul 1I I I PNS min CCP (mmhg) Drops I I I I I I PNS scs Venous ~MM I I I ~ 11, ii I I IiIIIIIIIIIIIIII; I I 1111VIIIII A CCP 100 (mmhg) 0 J Drops inliiiiliuifififi II I PNS IM 1.,1,mm scs _ Fig. 5. Effect of corpus cavernosal pressure (CCP) and arterial and venous flow through the corpus cavernosum during pelvic nerve stimulation (PNS, 10 Hz) and sympathetic chain stimulation (SCS, 3 Hz). SCS decreased arterial flow and CCP, and increased venous flow. stimulation, both flow from the 19 G needles and CCP decreased for the duration of the sympathetic chain stimulation (n = 3, Fig. 5). The decrease in CCP was accompanied by an increase in flow through the carotid shunt (n = 4). Effect of ACh and VIP on corpus cavernosal blood flow Single bolus injections of ACh ( ,sg, n = 4) or VIP (2-5 jug, n = 4) into the penile artery caused little or no change in CCP, despite decreases in PAP. Single bolus injections of ACh ( jg, n = 6) directly into the corpus cavernosum caused increases in CCP of between 50 and 100 % of that seen during pelvic nerve stimulation in the same animal. CCP remained elevated for 2-6 min post-injection.

10 84 C. J. CARATI, K. E. CREED AND E. J. KEOGH Infusion of ACh into the penile artery (330,ug min-m for 2 min) increased blood flow from the two 19 G needles in the corpus cavernosum, increased CCP and decreased blood flow through the carotid shunt (n = 4, Fig. 6). Similar effects were observed when VIP was infused (1-3 3 jug min-' for 2 min, n = 4), but the elevation of CCP tended to take longer to occur than with ACh infusion, and the effect was prolonged after cessation of the infusion (3-27 min for VIP, compared with min for ACh). Arterial flow Venous flow CCP 100 (mmhg) 0 J Drops ii n 1 I 1 II11, PAP 1004 (mmhg) ACh A CCP 120 Drops II1_ l1ii mint PAP (mmhg) VIP 2 min 26 min Fig. 6. Effect on corpus cavernosal pressure (CCP) and arterial and venous flow through the corpus cavernosum during 2 min close arterial infusions of acetylcholine (ACh, 330,ug min-') and vasoactive intestinal polypeptide (VIP, 0-3-1,ug min-'). Both ACh and VIP increased arterial flow, increased CCP and decreased venous flow. DISCUSSION The present experiments confirm studies by Purohit & Beckett (1979), who reported large differences between the elevated pressure of the corpora cavernosa and the corpus spongiosum during erection and intercourse in dogs. The lack of any exchange of pressure between the corpora cavernosa and the corpus spongiosum is contrary to anatomical descriptions of the canine penis. Christensen (1954) noted complete separation between the right and left corpora cavernosa, but found vascular connections between the corpora cavernosa and the corpus spongiosum (confirmed in our own unpublished studies). These connections have also been described in man (Wagner, Bro-Rasmussen, Willis & Nielsen, 1982). There must be a mechanism whereby such connections become occluded when corporeal pressure is elevated. The increase in pressure within the corpus spongiosum occurred immediately after the start of pelvic nerve stimulation, but was delayed in the corpora cavernosa by s. This gives direct support to previous studies which inferred that pelvic nerve stimulation caused rapid arterial dilatation and increased blood flow through the penis (particularly through the corpus spongiosum and glans), but this initially bypassed the corpora cavernosa (Andersson et al. 1984; Carati et al. 1987). Conversely, at the onset of pelvic nerve stimulation, there was usually an immediate

11 BLOOD FLOW DURING PENILE ERECTION decrease in corpus cavernosal pressure (Carati et al. 1987). In the present experiments flow through the carotid shunt also increased over the first s of pelvic nerve stimulation. This may reflect filling of the dilating sinusoidal spaces of the corpus cavernosum. In the unstimulated penis flow rate through the carotid shunt was rapid, indicating that venous resistance was low. On pelvic nerve stimulation flow through the shunt decreased and corpus cavernosal pressure rose. This could only occur if there was an increase in venous resistance. Arterial occlusion may also occur, since corpus cavernosal pressure far in excess of systemic pressure was recorded during saline infusion, while systemic and penile artery pressure were unaffected (see also Purohit & Beckett, 1979). The conclusion that there is an increase in venous resistance during erection is supported by our saline infusion studies and by those of others (Lue et al. 1984; Juenemann, Luo, Lue & Tanagho, 1986). Constant-volume infusions of up to 50 ml min-' were required to elevate and maintain corpus cavernosal pressure to equal mean systemic pressure, in the absence of stimulation. With elevated corpus cavernosal pressure during pelvic nerve stimulation, however, infusion of less than 1 ml caused large increases in pressure, above systemic arterial pressure. These results suggest that venous occlusion during erection must be actively controlled by pelvic nerve stimulation. Increased venous resistance followed increased corpus cavernosal pressure during pelvic nerve stimulation. It may be that the venous occlusive mechanism takes longer to initiate than arterial and pressure changes, or that the release of vasoactive substances requires its distension. The isolation of the corpora cavernosa from the corpus spongiosum was achieved by infusion of saline in the absence of neural or humeral stimulation, indicating that increases in pressure are sufficient to occlude at least some of the vascular connections of the penis. A recent anatomical explanation for this may be provided by Fournier, Juenemann, Lue & Tanagho (1987), who described subalbugineal venular plexuses in the crural region of the canine corpus cavernosum which were compressed between the tunica albuginea and the sinusoids in papaverine-induced erection. The relationship between arterial and venous flow and corpus cavernosal pressure at cessation of pelvic nerve stimulation is not clear. The increase in corpus cavernosal pressure was initially parallel to an increase in penile artery pressure, suggesting that they were connected by a low-resistance pathway. The rapid return of penile artery pressure to pre-stimulation levels may be due to rapid changes in vascular resistance of the corpus spongiosum (see also Carati et al. 1987). The changes following occlusion of the aorta during stimulation also suggest there was a low-resistance pathway between the penile artery and the corpus cavernosum during pelvic nerve stimulation. The resistance in this pathway must have increased within 1 min of cessation of stimulation since corpus cavernosal pressure remained elevated during aortic occlusion at this time. This could only occur if both arterial and venous outflow resistance were high. Thus it appears that venous changes during the loss of erection occurred after arterial changes. However, the return of arterial and venous flow and corpus cavernosal pressure to pre-stimulation levels shared similar time courses, with venous resistance more variable in its recovery. 85

12 86 C. J. CARATI, K. E. CREED AND E. J. KEOGH The arterial and venous changes during erection warrant investigation with directmeasurement techniques. However, in our preliminary experiments, complete isolation of the major arteries and veins supplying the corpora cavernosa proved difficult. Furthermore, occlusion of the penile arteries as they entered the corpora cavernosa failed to prevent the erectile response to pelvic nerve stimulation, indicating that blood also reached the corpora cavernosa through other arteries. Measurements of flow in the internal pudendal arteries or the dorsal penile veins (Lue et al. 1984; Andersson et al. 1984) are not specific to events in the corpora cavernosa, since these vessels also supply the corpus spongiosum. There are no reports of measurements of blood flow or volume of the canine corpus cavernosum. In our hydrogen probe experiments, flows in the flaccid and stable erect corpus cavernosum were equal, being approximately half that through adjacent striated muscle. During the first 30 s of erection there was an increase in corpus cavernosal pressure, and flow increased above that seen in the unstimulated corpus cavernosum (13 vs. 84 ml min-1 O g-1). This increase in flow may represent filling of the sinusoidal spaces, and increased flow through the corpus cavernosum prior to venous occlusion. It is interesting to note that some flow during elevated corpus cavernosal pressure was recorded with the hydrogen probe and in the carotid shunt experiments, indicating that venous occlusion is not complete at these pressures. The slight increase in flow following cessation of stimulation may reflect expulsion of blood from the corpus cavernosum. The more rapid rate of blood flow as corpus cavernosal pressure increased compared to when it decreased is consistent with the observation that vascular changes were quicker to occur after the onset of stimulation, compared to after its cessation (hence occurring at a quicker rate). The techniques employed to estimate arterial and venous changes in this study must be considered as qualitative only. However, the hydrogen probe experiments allow more quantitative estimates of blood flow rates through the canine corpus cavernosum. From our anatomical studies, we estimate canine corpus cavernosal volumes to be 2-5 ml, with little or no change during erection. The tissue: blood partition coefficient for hydrogen is 1 (Ackland, Bower & Berliner, 1964). Assuming homogeneity of blood flow through the cavernosum, and a blood and tissue density of one, blood flow rates in the flaccid and stable erect cavernosa can be estimated as less than 0-5 ml min-'. This rate increases by 6-5-fold during the first s of erection. Stimulation of the sympathetic innervation of the corpus cavernosum effected both arterial and venous changes. This sympathetic control could be exerted at the level of the blood vessels or the erectile tissue, or by inhibition of the pelvic nerve, perhaps at the pelvic plexus. Histochemical studies of the innervation of the mammalian penis have shown an adrenergic innervation of the cavernosal blood vessels and the smooth muscle of the erectile tissue (Baumgarten, Falek & Lange, 1969; Bock & Gorgas, 1977; McConnell, Benson & Wood, 1979). The sympathetic inhibition of the erectile process is cx-adrenergic in nature (Carati et al. 1987), and a- adrenergic agonists cause strong contractions of erectile tissue in vitro (Carati et al. 1985). It remains to be demonstrated how sympathetic activity would cause arteriolar constriction and decreased venous resistance. Infusions of VIP and acetylcholine caused changes in blood flow through the

13 BLOOD FLOW DURING PENILE ERECTION corpus cavernosum which mimicked those seen during pelvic nerve stimulation. Both substances caused relaxation of the in vitro erectile tissue of the canine corpus cavernosum (Carati et al. 1985), and their infusion caused increases in blood flow through the penis with a clear-cut, but submaximal, increase in penile volume (Andersson et al. 1984). These observations, the histochemical localization of VIP, and its release during erection, strongly suggest a role for VIP in penile erection (see Andersson et al. 1984; Carati et al. 1987). The role of acetylcholine is less clear, however. The pelvic nerve-induced increase in corpus cavernosal pressure could not be blocked by atropine (Carati et al. 1987), in contrast to the increase in penile (corpus spongiosum and glans) blood flow reported by Andersson et al. (1984). Acetylcholine may therefore be acting on atropine-resistant receptors to produce increases in corpus cavernosal pressure. We would like to thank Mr P. Burrows and Mrs V. Wakelam for expert assistance with the animals. The experimental work was conducted at the University Department of Surgery, University of Western Australia and the School of Veterinary Studies, Murdoch University, Western Australia. It was supported by the Impotence Study Group of Western Australia and the National Health and Medical Research Council of Australia. REFERENCES ACKLAND, K., BOWER, B. F. & BERLINER, R. W. (1964). Measurement of local blood flow with hydrogen gas. Circulation Research 14, ANDERSSON, P.-O., BLOOM, S. R. & MELLANDER, S. (1984). Haemodynamics of pelvic nerve induced penile erection in the dog: possible mediation by vasoactive intestinal polypeptide. Journal of Physiology 350, BAUMGARTEN, H. G., FALCK, B. & LANGE, W. (1969). Adrenergic nerves in the corpus cavernosum of some mammals. Zeitschrift fur Zellforschung und mikroskopische Anatomie 95, BOCK, P. & GORGAS, K. (1977). Morphology and histochemistry of the helicine arteries in the corpora cavernosa penis of mice. Archivum histologicum japonicum 40, CARATI, C. J., CREED, K. E. & KEOGH, E. J. (1987). Autonomic control of penile erection in the dog. Journal of Physiology 384, CARATI, C. J., GOLDIE, R. G., WARTON, A., HENRY, P. J. & KEOGH, E. J. (1985). Pharmacology of the erectile tissue of the canine penis. Pharmacological Research Communications 17, CHRISTENSEN, G. C. (1954). Angioarchitecture of the canine penis and the process of erection. American Journal of Anatomy 95, CREED, K. E., CARATI, C. J. & KEOGH, E. J. (1986). Penile blood flow during erection. Proceedings of the Australian Physiological and Pharmacological Society 17, 59P. DORR, L. D. & BRODY, M. J. (1967). Hemodynamic mechanisms of erection in the canine penis. American Journal of Physiology 213, ECKHARD, C. (1863). Untersuchungen iuber die Erection des Penis beim Hunde. In Beitrage zur Anatomie und Physiologie, III, ed. ECKHARD, C. Band Giessen. Cited in SJOSTRAND, N.O. & KLINGE, E. (1979). Acta physiologica scandinavica 106, FOURNIER, G. R., JUENEMANN, K-P., LUE, T. F. & TANAGHO, E. A. (1987). Mechanisms of venous occlusion during canine penile erection: an anatomic demonstration. Journal of Urology 137, JUENEMANN, K.-P., Luo, J.-A., LUE, T. F. & TANAGHO, E. A. (1986). Further evidence of venous outflow restriction during erection. British Journal of Urology 58, LANGLEY, J. N. & ANDERSON, H. K. (1895). The innervation of the pelvic and adjoining viscera. III. The external generative organs. Journal of Physiology 19, LUE, T. F., TAKAMURA, T., UMRAIYA, M., SCHMIDT, R. A. & TANAGHO, E. A. (1984). Hemodynamics of canine corpora cavernosa during erection. Urology 24, MCCONNELL, J., BENSON, G. S. & WOOD, J. (1979). Autonomic innervation of mammalian penis: A histochemical and physiological study. Journal of Neural Transmission 45,

14 88 C. J. CARATI, K. E. CREED AND E. J. KEOGH MILLER, C. L. & NG, K. C. (1981). The hydrogen clearance technique in measuring steady-state and transient changes in local blood flow. In Progres8 in Microcirculation Re8earch, ed. GARLICK, D., pp Sydney: University of New South Wales. NINOMIYA, H. (1980). The penile cavernous system and its morphological changes in the erected state in the dog. Japane8e Journal of Veterinary Science 45, PUROHIT, R. C. & BECKETT, S. D. (1979). Penile pressures and muscle activity associated with erection and ejaculation in the dog. American Journal of Phy8iology 231, WAGNER, G., BRO-RASMUSSEN, F., WiLLis, E. A. & NEILSEN M. H. (1982). New theory on the mechanism of erection involving hitherto undescribed vessels. Lancet i,