Catecholamines, oxytocin and milk removal in dairy cows

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1 Journal of Dairy Researh (1989), 6, Printed in Great Britain 167 Cateholamines, oxytoin and milk removal in dairy ows BY JURG W. BLUM*, DIETER SCHAMSf A RUPERT BRUCKMAIERf Institute of Animal Siene, Federal Institute of Tehnology, 8092 Zurih, Switzerland, ^Institute of Physiology, Tehnial University of Munih, 80 Freising - Weihenstephan, FRG (Reeived 24 Marh 1988 and aepted for publiation 3 November 1988) SUMMARY. Experiments were designed to study the effets of ateholamines on oxytoin responses and milk removal in dairy ows. Adrenalin, noradrenalin, dopamine, isoproterenol (a /?-adrenoeptor agonist), phentolamine (an a-adrenergi bloker) and propranolol (a /?-adrenergi bloker) were infused intravenously. In addition, adrenalin was infused together with phentolamine and/or propranolol. Infusions started 8 min before milking and lasted until the end of milking. In some ases eletroshoks (for s) were applied immediately before milking in the absene and presene of phentolamine and propranolol. Adrenalin, noradrenalin and dopamine redued milk removal, but only if administered in supraphysiologial amounts. The effet of adrenalin and eletroshoks on milk removal ould be inhibited by a-, but not by /?-adrenergi blokade. The effet of dopamine ould be inhibited only partly by phentolamine. Inhibition of milk removal was not mediated by redued oxytoin responses. Enhaned loal release of ateholamines from sympatheti nerves was presumably responsible for lowered milk removal in response to eletroshoks. Milk removal was failitated during a-adrenergi blokade and during /?-adrenoeptor stimulation. Oxytoin (OT) is primarily responsible for stimulation of milk ejetion (Linoln & Paisley, 1982; Gorewit et al. 1983; Lefourt & Akers, 1983). However, other fators an modify milk removal. This is partiularly true for the sympatheti nervous system (Mena et al. 1978, 1979; Goodman & Grosvenor, 1983; Gorewit et al. 1983; Shams et al. 1984). This an be explained by lose assoiation between OT ontaining neurons and the sympatheti nervous system of the brain (Aulsebrook & Holland, 1969; Moos et al. 1983; Seybold et al. 1978). Furthermore, there are lose onnetions between OT effets and the sympatheti nervous system in the mammary gland (Peeters et al. 1949; Lefourt & Akers, 1983). Thus, arteries, arterioles and smooth musles of the teat are sympathetially innervated (Peeters et al. 1949; Bernabe & Peeters, 1980). Moreover, arterial musles in the mammary gland are under ative sympatheti tone (Goodman & Grosvenor, 1983; Lefourt, 19826). In addition, rhythmial ontration of teat musles is due to hanges in the ativity of sympatheti nerves in the mammary gland (Sambraus, 1971; Lefourt, 1982a). The turgid state of the engorged mammary gland enhanes, whereas its relief * Present address: Department of Nutrition Pathology, Institute of Animal Breeding, Shool of Veterinary Mediine, University, 3012 Berne, Switzerland.

2 168 J. W. BLUM, D. SCHAMS A R. BRUCKMAIER by milking dereases the sympatheti tone in the gland (Mena et al. 1978; Lefourt, 19826). Administration of adrenalin and noradrenalin inhibits milk removal (Lefourt & Akers, 1983), but it is unlear whether this is a pharmaologial effet or whether it is also observed when only physiologial amounts are administered. In addition, it is not known whether this effet is due to inhibition of OT seretion (entral effet), or primarily situated in the mammary gland (peripheral effet). Also, it is not lear whether adrenalin and noradrenalin, transported through the vasular system to the mammary gland, are important for redution of milk let-down under onditions of emotional stress. However, marked inhibitory effets on milk let-down of loally released noradrenalin or of other amines an be expeted under onditions of emotional stress. Alpha- and /?-adrenergi reeptors are loated on arterial and teat (sphinter) smooth musle ells (Peeters & De Bruyker, 197; Peeters et al. 1977; Bernabe & Peeters, 1980; Vandeputte-Van Messom et al. 1984a). Alpha-adrenoeptor agonists stimulate ontration, whereas /?-adrenoeptor agonists and a-adrenergi bloking agents ause relaxation of these musles (Peeters & De Bruyker, 197; Peeters et al. 1977; Bernabe & Peeters, 1980). The effets of adrenalin and noradrenalin are inhibited by a-adrenergi bloking agents, at least in attle (Bernabe & Peeters, 1980; Vandeputte-Van Messom et al. 1982, 1984a). There is no evidene for diret sympatheti innervation of myoepithelial ells, but adrenalin and noradrenalin inhibit the ontratile response to OT (Lefourt & Akers, 1982). Furthermore, amines other than noradrenalin and adrenalin, suh as serotonin, an modify milk let-down (Vandeputte-Van Messom et al ). Surprisingly, effets on milk let-down of dopamine, whih is highly onentrated in mast ells of ruminants (Falk et al. 1964; Blum et al. 1980) and may be released under ertain onditions, have not yet been studied to our knowledge. Thus, ateholamines an influene milk removal. Effets may be mediated by alteration of OT release and by hanges in blood flow and therefore the amount of OT presented to myoepithelial ells (Dhondt et al. 1973; Gorewit & Aromando, 198; Gorewit & Sott, 1986). Cateholamines ould hange the sensitivity and/or responsiveness of myoepithelial ells to OT. In addition they ould diretly alter the tension of smooth musles surrounding mammary duts and in the teat and, as a onsequene, the transport of milk within the mammary gland. We have studied the onditions under whih ateholamines, transported to the mammary gland via the vasular system, ould modify milk removal in dairy ows. MATERIALS A METHODS Three series of experiments were performed during different periods of the year. Fifteen dairy ows, five animals/experimental series, were available. They were German Braunvieh x \ to \ Brown Swiss. Cows belonged to the Institute of Physiology, Tehnial University of Munih, Weihenstephan. The herd average annual yield was 7000 kg. The animals weighed kg. They were in months 3-6 of their first to ninth latation. The animals had free aess to hay and were fed silage and onentrate twie daily. To equalize a.m. and p.m. milk yields milking started at and 18.00, i.e. at 12 h intervals, beginning about 1 week before (period of adaptation) and during the experimental period. The milking system and methods of measuring parameters of milk removal have been desribed in detail (Mayer et al. 1984). Milking was started after 1 min of manual prestimulation. The main milking period was finished when

3 Cateholamines, oxytoin and milk removal 169 milk flow rate fell below 0-2 kg/min. In the ensuing period milk was stripped until milk flow fell again below 0-2 kg/min. Sixteen experimental protools were followed: ontrol experiments (no treatments); infusions of NaCl, adrenalin (in two doses), noradrenalin (in two doses), isoproterenol (a /S-adrenergi agonist) or phentolamine (an a-adrenergi bloker) alone; infusions of phentolamine ombined with propranolol (a/?-adrenergi bloker), of phentolamine with adrenalin, of propranolol with adrenalin, of phentolamine with propranolol or with adrenalin and of phentolamine with dopamine. Furthermore, eletri shoks (eletroshoks) were applied for s alone or ombined with infusions of phentolamine and propranolol. Amounts infused are given in Table 1. Intravenous steady state infusions by pumps were started 8 min before milking and were stopped at the end of the main milking period. Indwelling atheters were inserted 1-2 d before the experiments into the jugular vein. Eletroshoks were applied by use of an eletri prod (42 V/impulse) immediately before prestimulation. Adrenalin bitartrate and noradrenalin bitartrate were purhased from Fluka AG, Buhs, Switzerland; dopamine-hcl from Hausmann Laboratories, St. Gallen, Switzerland; and isoproterenol-hcl from Winthrop Produts, Malesfield, Cheshire, UK. methanesulphonate was donated by Ciba-Geigy AG, Basle, Switzerland and D,1-propranolol by ICI, Surbiton, Surrey, UK. All substanes were dissolved in 0-92% NaCl shortly before use and kept on ie in light-proteted bottles during the experiments. Citri aid (300 mg/1) was added to propranolol solutions. Blood samples ( ml) were obtained through a atheter, inserted 1-2 d before the experiments, from the jugular vein ontralateral to the one used for the infusions. Samples were obtained at, 9, 8, 2, 1 and 0 min before milking; at 0-, 1, 1-, 2, 2-, 3, 4 min and then every min during the main period of milking; and at 2 and min after the end of milking. Blood was immediately transferred to tubes ontaining heparin whih were left on ie and then entrifuged at 4 C within 1-30 min for the separation of plasma. Plasma was stored at 20 C in multiple aliquots in plasti ups until determination of adrenalin, noradrenalin or OT. Adrenalin and noradrenalin were measured radioenzymially (Blum el al. 1980) and OT radioimmunologially (Shams, 1983) after extration with SEP-PAK C 18 artridges. Statistial analysis was performed by Wiloxon-Test and by linear regression analysis. Data are presented as means + s.e.m. RESULTS Milking parameters were similar in ows infused with NaCl and in ontrol animals without infusions. OT responses were in the normal range (Table 1) exept for one animal whih reated exessively (4pg/ml; data not used). Levels of adrenalin and noradrenalin did not hange before and during milking in ontrol experiments (Fig. 1). During adrenalin infusions, onentrations of adrenalin inreased markedly within minutes in a dose-dependent manner (Table 1). OT responses were not altered by adrenalin administration (Fig. 2). Total and main yield dereased (signifiantly for 021 fig adrenalin/kg) (Table 1). Total and main milking times were markedly and signifiantly shortened and, in addition, mean and peak flow rates were dereased (signifiantly for 0-21 /tg adrenalin/kg; P < 0-0). During noradrenalin infusions, onentrations of noradrenalin inreased within minutes in a dose-dependent manner, the rise being absolutely and relatively smaller

4 170 J. W. BLUM, D. SCHAMS A R. BRUCKMAIBR a. 20 B <D < I a eg a -+> 20 z Eletroshok s min Fig. 1. Mean hanges in adrenalin and noradrenalin before and after eletroshoks ( ; immediately before prestimulation, i.e. 1 min before milking) in the absene (O) aid presene (0) of phentolamine (/tg/kg.min) and propranolol (/ig/kg.min), i.v.-infused from 8 min before until the end of milking, ompared to ontrols ( + ). than that of adrenalin during adrenalin infusions (P < 0-0; Table 1, Fig. 3). OT responses were not signifiantly affeted by noradrenalin administration. Total and main yield were dereased with 0-21 /ig noradrenalin/kg (P < 0-0). Peak flow rate dereased (P < 0-0). Infusions of dopamine aused a signifiant derease of total and main yield (P < 0-0) as well as yield by stripping (P < 0-0). OT responses were not hanged (Table 1) but OT inreased exessively in one ow (242 pg/ml; data not used). During infusions of the /?-adrenoeptor agonist isoproterenol total and main yield inreased and milk flow rate inreased, but total and main milking time dereased, although effets were not signifiant (Table 1). Time to peak flow was signifiantly redued (P < 0-0). OT responses were in the normal range exept for one ow whih reated exessively (234 pg/ml; data not used). Total and main yield inreased 1 and 19% respetively, during a-adrenergi blokade with phentolamine (Table 1), whereas yield by stripping was half that in ontrols, but none of these effets were signifiant. However, time to peak flow was redued signifiantly (P < 0-0). OT responses were in the normal range exept for one ow whih responded exessively (240 pg/ml; data not used). The ombined administration of the a- and /?-adrenergi bloking agents phentolamine and propranolol did not hange milking parameters as well as adrenalin and noradrenalin levels (Table 1). OT responses were not modified, but in experiments with dopamine ombined with phentolamine, OT response was exessive in one ow (208 pg/ml; data not used). During adrenalin infusion ombined with

5 Cateholamines, oxytoin and milk removal ,6 8 lomin Fig. 2. Changes in milk flow ( ) and blood levels of oxytoin ( ) in a ow in the presene of exogenous (a), adrenalin (0-21 //g/kg.min) ombined with propranolol (/(g/kg.min); (ft), adrenalin (0-21 //g/kg.min) together with phentolamine (/<g/kg.min); (), adrenalin alone (0-21 /(g/kg.min) or (rf), in the absene of exogenous adrenalin and bloking agents (ontrol). Adrenalin, propranolol and phentolamine were i.v. infused from 8 min before until the end of milking. Prestimulation was from 1 min until 0 min ( ). Arrows (f ) indiate the start and end respetively, of the main milking period E 30 * 20 'o o - X O min Fig. 3. Changes in milk flow ( ) and blood levels of oxytoin ( ) in the presene of exogenous noradrenalin ((a) 021 and (6) 006/(g/kg.min, i.v. infused from 8 min before until the end of milking) and () in the absene of exogenous noradrenalin (ontrol). For further details see Fig. 2.

6 Table 1. Effets of adrenalin, noradrenalin, dopamine, isoproterenol, phentolamine, propranolol, eletroshok or NaCl (alone or ombined) on oxytoin response, blood plasma levels of adrenalin and noradrenalin and on parameters of milk removal (means + s.e.m.) Control NaCl Adrenalin Adrenalin Noradrenalin Noradrenalin Dopamine Isoproterenol + propranolol + adrenalin Propranolol + adrenalin + propranolol + adrenalin + dopamine Eletroshok + propranolol + eletroshok /tg/kg.min n Oxytoin a, pg/ml ± ± ± ±7 8±2 Adrenalin", pg/ml * * 62± * * * t t Noradrenalin, pg/ml ± * * ± ±4 36 ±77 t t Total yield, kg -2 ±0-7 1 ± ± 4-3 ±0-9* ±1-0* * -8 ±0-6 ll-7± ± ±1-4* -3 ± ± ±0-8 Stripping, kg ±01 0- ±01 0- ± ± ±00* 0-4 ± ± ± ± ±00* 0-9 ±0-3* * time, min ±0-6 - ±0-3-7 ±0-4* -6 ± ± ±0-9 - ± ±0-8* -9 ± ± ±0-7 1 ±0-7 time, min -3 ±0-6 1 ±0-4-4 ±0-2-6±0-3* 4-6 ± ± ± ± ± ±0-6* 4-6 ± ± ±0-6* rate, kg/min 20 ± ±0-2 l-2±0-2* ±0-2 l-2± ± ± ±0-2* 2-4±0-4 l-7±0-3 l-3±0-2* 21 ±0-4 peak flow min ( 9 ±0-2 l±0-2* * >8±0-2* l-2±0-l* 1-8 ± ±0-6 l±0-3* 11 ±0-2* 1-4 ±0-3 l-8± ±0-2* ±0-3* l-3±0-2* a, Means from 0 to 2 min of milking; *, means from 8 min before to the end of milking. Signifiane of differene from ontrol experiments :*,/>< 00; no symbol, P > 00., Not done; t> see Fig. I. Statistial analysis of eah experimental series was based on its own ontrol. to td fel CO a M> g CO > fd a i f fef

7 Cateholamines, oxytoin and milk removal 173 I a 30 (a) 20 mi omj.1-1.1a 1 - Mf 30 <b) 20' i A r\ * - «f 30 3 () 20 v 2 ' 1 O-n-n-rrfX min I r E o Fig. 4. Changes in milk flow ( ) and blood levels of oxytoin ( ) in a ow with appliation of eletroshok (j, lasting for s and applied immediately before prestimulation, i.e. 1 min before milking); (a), in the presene of phentolamine (/«g/kg.min) and propranolol ( /tg/kg.min), both i.v. infused together from 8 mill before milking until the end of milking; (6), with appliation of an eletroshok alone; (), without eletroshok (ontrol). For further details see Fig. 2. phentolamine and/or propranolol, levels of adrenalin inreased to the same extent as in the absene of bloking agents (Table 1; Fig. 2). OT responses, too, were not signifiantly affeted by ombined infusions of adrenalin with bloking agents. Whereas total and main yield slightly but not signifiantly inreased during adrenalin and phentolamine infusions, total and main yield were dereased during the administration of adrenalin and propranolol (P < 0-0). In addition, total and main milking time, mean flow rate and time of peak flow were signifiantly dereased when adrenalin and propranolol were infused (P < 0-0), but were not hanged signifiantly during ombined adrenalin and phentolamine administration. When adrenalin was infused together with propranolol and phentolamine, milking parameters were not signifiantly hanged. When dopamine was infused together with phentolamine, yield by stripping was half that in ontrols {P < 0-0) and time to peak flow was also markedly redued (P < 0-0). Eletroshoks were followed by a rapid, but transient, inrease partiularly of adrenalin and less of noradrenalin (P < 0-0; Fig. 1). OT responses were not hanged by eletroshoks alone or eletroshoks in the presene of phentolamine and propranolol (Fig. 4). Eletroshoks only slightly dereased total yield, but signifiantly dereased main yield (P < O0), whereas yield by stripping was more than doubled ompared to ontrols (P < 0-0) (Table 1; Fig. 4). Main flow rate was redued (P < 0-0), whereas time to peak flow beame markedly greater, but not signifiantly so. Infusions of the a- and /?-adrenergi bloking agents (phentolamine and propranolol) partly inhibited effets of eletroshoks on total and main yield. However, yield obtained by stripping remained elevated (P < 0-0). Main yield and time to peak flow were signifiantly shortened in the presene of blokers (P < 0-0).

8 174 J. W. BLUM, D. SCHAMS A R. BRUCKMAIBR DISCUSSION Milking parameters in ontrols were in a range omparable to previous studies (Mayer et al. 1984; Shams et al. 1984). Main yield was losely related to main flow rates whih thus determined total yield. Eletroshoks markedly inreased yields by stripping and thus differed from effets of other treatments. Although there were onsistent effets within experimental protools, individual variability was onsiderable, espeially in response to eletrial shoks (not shown). Oxytoin onentrations inreased during prestimulation and milking, in aordane with previous reports (Mayer et al. 1984; Shams et al. 1984). In experimental series 2, 7, 8, 9 and 14 OT responses were exessive in one ow and therefore exluded while in the remaining animals OT was in the normal range. The magnitude of OT responses during the first 2 min of milking was not related to parameters of milk removal, as in previous studies (Sagi et al. 1980; Gorewit et al. 1983; Shams et al. 1984). Our results therefore demonstrate that OT responses to prestimulation and milking were not modified by the administration of various ateholamines, by bloking agents, or by eletroshoks. Thus, the administered ateholamines and eletroshoks mediated their effets on milk removal independent of irulating OT. Similarly, OT responses in ows were not hanged during noradrenalin administration (Lefourt & Akers, 1982). Exessive amounts of adrenalin used were possibly responsible for dereased OT responses reported by Gorewit & Aromando (198). In our study we found no evidene for a delayed OT release after treatment of ows with eletrial urrent as found by Henke Drenkard et al. (198). Levels of adrenalin and noradrenalin did not hange during normal milking in this study. The sympatheti reflex during milking (Lefourt, 19826; Goodman & Grosvenor, 1983) is therefore not aompanied by hanges in blood ateholamine levels. However, levels of adrenalin and noradrenalin inreased immediately after eletroshoks in our study, in ontrast to Lefourt & Akers (1984) and Lefourt et al. (198). Eletroshoks dereased milk removal even though blood levels of adrenalin and noradrenalin reahed after this treatment were muh smaller than those reahed during infusions of adrenalin and noradrenalin. Therefore, fators other than irulating ateholamines must have been responsible for inhibition of milk letdown. Cateholamines and possibly other substanes, released loally in the udder following eletrial shoks, possibly aused dereased milk let-down. Blood levels reahed during infusions of adrenalin and noradrenalin were greatly above those reahed after eletroshoks or during strenuous treadmill exerise (Blum et al. 1979). Only with 0-21 /ig adrenalin or noradrenalin/kg was milk removal dereased signifiantly. Thus, only supraphysiologial blood levels of adrenalin and noradrenalin dereased milk let-down, in ontrast to Lefourt & Akers (1983). Gorewit & Aromando (198) suggested that adrenalin exerts its inhibitory effets on milk removal peripherally by preventing OT from reahing myoepithelial ells. Based on our own studies this annot be the only effet explaining the inhibition of milk let-down (R. Brukmaier, unpublished observations). Adrenalin was a more potent inhibitor of milk removal than noradrenalin. This was possibly the onsequene of higher blood levels of adrenalin ompared to noradrenalin, even though the same amounts were administered. It is explained by faster learane from the irulation of noradrenalin than of adrenalin (Frohli & Blum, 1988). To our knowledge it has not been demonstrated before that dopamine auses a derease in milk removal. Dopamine differed from adrenalin and noradrenalin by

9 Cateholamines, oxytoin and milk removal 17 dereasing espeially yield obtained by stripping. It seems therefore to be partiularly effiient in ausing milk retention. Dopamine in the amounts used has marked effets on irulation and endorine systems, even though it is extremely rapidly destroyed in bovine blood plasma (Blum et al. 1980; Blum, 1984; Frohli & Blum, 1988). Dopamine released loally from mast ells, where it is highly onentrated (Palk et al. 1964; Blum et al. 1980), may be responsible for dereased milk removal under ertain pathologial onditions. The y?-adrenoeptor agonist isoproterenol tended to improve milk removal and partiularly enhaned time to peak flow, as reported by Bernabe & Peeters (1980), Hamann (1981) and Bernabe & Riordel (198a, 6) who also used isoproterenol or lenbuterol. Peeters et al. (1977) and Bernabe & Peeters (1980) demonstrated relaxation of the smooth musles of the teats as well as dereased spontaneous motility in response to isoproterenol. Thus, /?-adrenoeptor agonists seem to enhane milk flow rates by permitting greater opening of the teat anal. The a- and /?-adrenergi reeptor blokers, phentolamine and propranolol, were administered to define reeptors mediating effets of ateholamines on milk removal. alone partiularly enhaned time of peak flow, in aordane with Dhondt et al. (1973), Bernabe & Peeters (1980) and Bernabe & Riordel (198a). In previous studies with ows, administration of the same amounts of phentolamine as in the present investigation was assoiated with a derease of systemi blood pressure and an inrease of the heart rate, of nonesterified fatty aid and of noradrenalin levels (Blum et al. 1978; Frohli & Blum, 1988). These ardiovasular and metaboli effets are typially seen also during the administration of /?-adrenoeptor agonists. Improvement in milk removal an be explained by blokade of a-adrenergi reeptors, an enhaned release of noradrenalin, interation of noradrenalin with /?- adrenergi reeptors on smooth musle ells. This would be followed by relaxation of musle ells of duts and teats. even reversed inhibitory effets of exogenous adrenalin on milk removal, by removing a-adrenergi and possibly also by unmasking /?-adrenergi omponents of adrenalin. These results are in aordane with those of Bernabe & Peeters (1980), Mielke (1981) and Vandeputte-Van Messom et al. (1982, 1984a). On the other hand, the ombined a- and /?-adrenergi blokade, with or without exogenous adrenalin, did not modify parameters of milk removal. In this situation propranolol presumably redued milk removal by bloking effets via /ff-adrenergi reeptors of adrenalin or noradrenalin. Milk removal by adrenalin was inhibited by propranolol to the same extent as in its absene, demonstrating that blokade of /^-reeptors does not inhibit the effet of adrenalin. This is in aordane with Bernabe & Peeters (1980). As a-adrenergi blokade only partly suppressed effets of dopamine, this suggests that dopamine mediates its inhibitory effets on milk removal also via speifi (dopaminergi) reeptors. Similarly, ombined a- and /?-adrenergi blokade did not suppress ompletely inhibitory effets of eletroshoks on milking parameters. In partiular, yield by stripping remained inreased. Besides adrenalin and noradrenalin, whose release was transiently enhaned, additional fators must have been responsible for inhibition of milk removal following eletroshoks. Our study supports findings of others that adrenalin and noradrenalin mediate their inhibitory effets on milk removal through interation with a-adrenergi reeptors loated in the udder, most likely on smooth musle ells of duts and teats. Effets of dopamine may be mediated by speifi reeptors, at least in part. It appears that effets of adrenalin, noradrenalin and dopamine are diret and independent of OT, whose release was not modified by treatments. Our data indiate

10 176 J. W. BLUM, D. SCHAMS A R. BRUCKMAIER that adrenalin and noradrenalin, irulating in inreased amounts in blood under onditions of emotional stress, are of small importane ompared to noradrenalin and possibly dopamine or other substanes, released loally in the udder under stress and other onditions. Alpha-adrenergi bloking agents may be used to remove inhibition of milk let-down under stress onditions and in udders with injured teats. This study was in part supported by the Shweizerishe Zentralverband fur Milhwirtshaft and the German Researh Foundation. We thank Mr A. Prokopp and Mr J. D. Steib for their great help in animal experiments. The tehnial performane of hormone assays by Mrs R. Admaty, Mrs C. Braun and Mr W. Moses is greatly appreiated. We thank Dr H. Worstorff and Mr A. Prediger for their help in establishing the milking equipment and Mr E. Lehmann and Mr H. Ronge for their ontribution in the statistial analysis of the data. REFERENCES AULSBBROOK, L. H. & HOLLA, R. C Central inhibition of oxytooin release. Amerian Journal of Physiology BERNABE, J. & PBETERS, G Studies on the motility of smooth musles of the teats in latating ows. Journal of Dairy Researh BERNABE, J. & RICORDEL, M.-J. 198a [Changes in the onditions of milk extration aused by a /^-reeptor agonist (isoprenaline) injeted into the jugular vein during mahine milking of the ow.] Reprodution, Nutrition, De'veloppement BERNABE, J. & RICORDEL, M.-J [Effets of adrenalin and phenylephrine on milk extration during mahine milking of ows.] Reprodution, Nutrition, De'veloppement BLUM, J. W Insulin suppressive effets of aminotetraline analogs and of dopamine. European Journal of Pharmaology BLUM, J. W., BIANOA, W., NAF, F., KUNZ, P., FISCHER, J. A. & DA PRADA, M Plasma ateholamine and parathyroid hormone responses in attle during treadmill exerise at simulated high altitude. Hormone and Metaboli Researh BLUM, J. W., GUILLEBEAU, A., BINSWANOER, U., KUNZ, P., DA PRADA, M. & FISCHER, J. A Effets of alpha-adrenergi stimulation and blokade on plasma parathyroid hormone onentrations in ows. Ada Endorinologia BLUM, J. W., KUNZ, P., FISCHER, J. A., BINSWANGER, U., LICHTENSTEIOER, W. & DA PRADA, M Parathyroid hormone response to dopamine in attle. Amerian Journal of Physiology 239 E2-E264 DHOT, G., HOUVENAGHEL, A., PEETERS, G. & VERSCHOOTEN, F Influene of vasoative hormones on blood flow through the mammary artery in latating ows. Arhives Internationales de Pharmaodynamie FALCK, B., NYSTEDT, T., ROSENQREN, E. & STENFLO, J Dopamine and mast ells in ruminants. Ada Pharmaologia et Toxiologia FROHLI, D. & BLUM, J. W Blood levels, learane rates and effets of epinephrine and norepinephrine on insulin and metabolites during alpha- and beta-adrenergi blokade in attle in vivo, and in vitro degradation of dopamine in bovine blood. Ada Endorinologia GOODMAN, G. T. & GROSVENOR, C. E Neuroendorine ontrol of the milk ejetion reflex. Journal of Dairy Siene GOREWIT, R. C. & AROMAO, M. C. 198 Mehanisms involved in the adrenalin-indued blokade of milk ejetion in dairy attle. Proeedings of the Soiety for Experimental Biology and Mediine GOREWIT, R. C. & SCOTT, N. R Cardiovasular responses of ows given eletrial urrent during milking. Journal of Dairy Siene GOREWIT, R. C, WACHS, E. A., SAOI, R. & MERRILL, W. G Current onepts on the role of oxytooin in milk ejetion. Journal of Dairy Siene HAMANN, J [The influene of a /? 2 -mimeti ative substane (Planipart) on the milking behaviour of ows.] Tierarztlihe Umshau HENKE DRENKARD, D. V., GOREWIT, R. C, SCOTT, N. R. & SAGI, R. 198 Milk prodution, health, behavior, and endorine responses of ows exposed to eletrial urrent during milking. Journal of Dairy Siene LEFCOURT, A. M. 1982a Rhythmi ontrations of the teat sphinter in bovines: an expulsion mehanism. Amerian Journal of Physiology 242 R181-R184 LEFCOURT, A. M Effet of teat stimulation on sympatheti tone in bovine mammary gland. Journal of Dairy Siene

11 Cateholamines, oxytoin and milk removal 177 LEKCOURT, A. M. & AKEBS, R. M Endorine responses of ows subjeted to ontrolled voltages during milking. Journal of Dairy Siene LEPCOURT, A. M. & AKERS, R. M Is oxytoin really neessary for effiient milk removal in dairy ows? Journal of Dairy Siene LEFCOURT, A. M. & AKERS, R. M Small inreases in peripheral noradrenaline inhibit the milk-ejetion response by means of a peripheral mehanism. Journal of Endorinology LEFCOURT, A. M., AKERS, R. M., MILLER, R. H. & WBINLA, B. 198 Effets of intermittent eletrial shok on responses related to milk ejetion. Journal of Dairy Siene LINCOLN, D. W. & PAISLEY, A. C Neuroendorine ontrol of milk ejetion. Journal of Reprodution and Fertility MAYER, H., SOHAMS, D., PROKOPP, A. & WORSTORFF, H Effets of manual stimulation and delayed milking on seretion of oxytoin and milking harateristis in dairy ows. Milhwissenshaft MGNA, F., PACHECO, P., AGUAYO, D., CLAPP, C. & GROSVENOR, C. E A rise in intramammary pressure follows eletrial stimulation of mammary nerve in anesthetized rats. Endorinology MGNA, F., PACHECO, P., AGUAYO, D., MARTINEZ, G. & GROSVENOR, C. E Reflex regulation of autonomi influenes upon the oxytoin-indued ontratile response of the mammary gland in the anesthetized rat Endorinology MIELKB, H [Reent results from studies on inhibited milk ejetion.] Monatshefle fur Veterindrmedizin Moos, F., FREU-MERCIER, M. J. & RICHARD, P [Aminergi and peptidergi ontrol of neuroseretory bursts in oxytoin ells during sukling.] In Multihormonal Regulations in Neuroendorine Cells pp (Eds A. Tixi'er-Vidal and P. Rihard) Paris: INSERM {Collogues 1NSERM No. 1, 1982) PEETERS, G., COUSSENS, R. & SIERENS, G Physiology of the nerves in the bovine mammary gland. Arhives Internationales de Pharmaodynamie el de The'rapie PEETERS, G. & DE BRUYCKER, R. 197 Influene of sympathomimeti drugs on the motility of bovine teat musles. Journal of Dairy Researh PEETERS, G., PETRE, P. & QUINTELIER, W Nature of adrenoeptor sites in bovine teat musles. Naunyn- Shmiedebergs Arhives of Pharmaology SAOI, R., GOREWIT, R. C, MERRILL, W. G. & WILSON, D. B Premilking stimulation effets on milk performane and oxytoin and prolatin release in ows. Journal of Dairy Siene SAMBRAUS, H. H [Rhythmi ontrations of the bovine teat.] Zentralblatt fitr Veterindrmedizin 18A SCIIAMS, D Oxytoin determination by radioimmunoassay. III. Improvement to subprogram sensitivity and appliation to blood levels in yli attle. Ada Endorinologia SCIIAMS, >., MAYER, H., PROKOPP, A. & WORSTORFF, H Oxytoin seretion during milking in dairy ows with regard to the variation and importane of a threshold level for milk removal. Journal of Endorinology SEYBOLD, V. S., MILLER, J. W. & LEWIS, P. R Investigation of a dopaminergi mehanism for regulating oxytoin release. Journal of Pharmaology and Experimental Therapeutis VAEPUTTE-VAN MESSOM, G., BERNABE, J., BURVENICH, C. & PEETERS, G Effets of a-bloking agents on teat motility in latating ows. Arhives Internationales de Pharmaodynamie VAEPUTTE-VAN MESSOM, G., BERNABE, J., BURVENICH, C. & PEETERS, G Effet of prazosin on the funtion of the teat sphinter in latating ows. Journal of Dairy Researh VAEPUTTE-VAN MESSOM, G., BURVENICH, C. & PEETERS, G. 1984a Ation of epinephrine on the funtion of the teat sphinter in the latating ow. Amerian Journal of Veterinary Researh