Steady State Aldosterone Dose-Response Relationships

Transcription

1 138 CIRCULATION RESEARCH VOL. 40, No. 2, FEBRUARY Tada M, Kirchberger MA, Repke DI, Katz AM: The stimulation of calcium transport in cardiac sarcoplasmic rcticulum by adenosine 3':5'-monophosphate-dependent protein kinase. J Biol Chem 249: , Nayler WG, Dunnett J, Berry D: The calcium accumulating activity of subcellular fractions isolated from rat and guinea pig heart muscle. J Mol Cell Cardiol 7: , Dhalla NS, McNamara DB, Sulakhe PV: Excitation-contraction coupling in heart. V. Contraction of mitochondria and sarcoplasmic reticulum in the regulation of calcium concentration in the heart. Cardiology 55: , Katz AM, Corkedale S, Kirchberger MA, Shigekawa M: Cyclic AMP dependent protein kinase effects on calcium efflux from canine cardiac sarcoplasmic reticulum (SR) (abstr). Fifth International Biophysics Congress, Copenhagen, August 4-9, 1975, p Weber A: Regulatory mechanisms of the calcium transport system of fragmented rabbit sarcoplasmic reticulum. I. The effect of accumulated calcium on transport and adenosine triphosphate hydrolysis. J Gen Physiol 57: 50-70, Katz AM, Dunnett J, Repke DI, Hasselbach W: Control of calcium permeability in the sarcoplasmic reticulum. FEBS Lett 67: , 1976 Steady State Aldosterone Dose-Response Relationships DAVID B. YOUNG AND ARTHUR C. GUYTON ALTHOUGH several decades of clinical observation and experimental investigation have proved qualitatively that aldosterone is an important participant in the control of extracellular electrolytes and volume, we have been unable to find in the literature data describing quantitatively the long-term dose-response effects of aldosterone. Therefore, the following study was undertaken to generate a series of steady state dose-response curves relating the rate of aldosterone infusion into adrenalectomized dogs with plasma concentrations of sodium and potassium, sodium space, mean arterial pressure, and plasma renin activity. Methods Long-term experiments were carried out in a group of five male mongrel dogs whose weights averaged 22.5 kg. Several weeks prior to the start of the study the dogs were adrenalectomized and cannulas were implanted into the aorta and vena cava through the femoral vessels. 1 Following recovery a portable infusion pump designed in our laboratory was connected to the venous cannula, and al- From the Department of Physiology and Biophysics, University of Mississippi School of Medicine, Jackson, Mississippi. Supported by Grant HL from the National Institutes of Health. Address for reprints: Dr. David B. Young, Department of Physiology and Biophysics, University of Mississippi School of Medicine, 2500 North State Street, Jackson, Mississippi Received February 2, 1976; accepted for publication September 14, SUMMARY The steady state effects of different infusion rates of aldosterone on plasma concentrations of sodium and potassium, plasma renin activity, sodium space, and mean arterial pressure were determined. Measured amounts of aldosterone were infused continuously into adrenaleclomized dogs for 13 weeks. Four rates of aldosterone administration were used: 16 ± 1 fig/day, 48 ± 3 pig/day (approximately the normal secretory rate for 22-kg dogs on a daily sodium intake of 27 meq, potassium intake of 27 meq), 91 ± 4 fig/day, and 219 ± 10 ftg/day. Each rate of infusion was continued until the dogs were in sodium and potassium balance and measured variables were steady. Decreasing the rate of aldosterone administration below normal led to sharp decreases in plasma sodium concentration and sodium space, while raising the rate above normal had little effect. Plasma potassium concentration varied inversely and significantly with changes in aldosterone administration over the entire range of rates. Plasma renin activity rose extremely rapidly as the level of aldosterone fell below normal and went to zero at aldosterone infusion rates slightly above normal. Arterial pressure increased as aldosterone rose above normal but did not fall below normal at subnormal aldosterone levels, probably because of the pressor effects of simultaneously generated angiotensin II. dosterone (CIBA) and methylprednisolone (Solu-Medrol, Upjohn) were infused in distilled water continuously 24 hours/day, 7 days a week, for 13 weeks. The rate of flow from the pump was 15 ml/day throughout the study. The rate of aldosterone infusion was increased in four steps, each step lasting at least 2 weeks. Each infusion level was continued until the dogs had reached new sodium and potassium balances and all measured variables were steady. The first rate of aldosterone infusion was 16 ± 1 (SE) /ig/day, followed successively by rates of 48 ±3 /xg/ day, 91 ± 4 /Ag/day, and 219 ± 10 ng/day. One dog died after the completion of the second infusion level. At all four levels, 1 mg/day of methylprednisolone was infused along with aldosterone. This dose of synthetic glucocorticoid has essentially no mineralocorticoid effect and was adequate to keep the dogs in good health. Throughout the experiment the dogs were housed in metabolic cages and fed a diet containing 27 meq/day each of sodium and potassium. Between 9 a.m. and 12 noon each day during the experiment the dogs were brought into the laboratory in which they had been trained to lie quietly unrestrained. Arterial blood was drawn from the indwelling cannula into syringes containing several microliters of heparin (1,000 U/ml). Plasma for electrolyte analysis was obtained by immediately centrifuging the blood in a high speed centrifuge for a period of less than 5 minutes, then removing the plasma. All electrolyte determinations were made with an Instrumentation Laboratory model 343 flame photometer. For

2 139 ALDOSTERONE DOSE-RESPONSE/Voung and Guyton plasma measurements the flame photometer was calibrated using a standard solution with a viscosity of 1.8 centipoises (cp), the approximate viscosity of plasma. Plasma sodium and potassium concentrations were determined each day during the study; when these values reached a constant level and the dogs were in approximate sodium and potassium balance, the following measurements were made for each of the next 3 days: plasma and urine sodium-potassium concentration, plasma renin activity, mean arterial pressure, body weight, and on the last day of infusion of each level of aldosterone, 22Na space (used to estimate changes in extracellular fluid volume). Plasma renin activity was estimated by the technique of Haber et al.2 Briefly, arterial blood was placed in chilled test tubes containing ethylenediaminetetraacetic acid (EDTA)-sodium immediately after being drawn. The plasma was separated by centrifugation and stored frozen until the renin activity was determined by measuring radioimmunologically the rate of angiotensin I generation in the inhibitor-treated plasma. Mean arterial pressure was measured from the indwelling arterial cannula connected to a Statham model 23Ac pressure transducer, the output of which was recorded on a Grass polygraph. The transducer was calibrated using a mercury manometer before each measurement period. Sodium space was calculated from the dilution of 10 /xci of 22Na injected intravenously 3 hours prior to sampling.3"5 The dogs were fed after the completion of daily data collection. Results Figure 1 shows the levels of urinary sodium and potassium excretion at the end of the four levels of aldosterone infusion. The group means ± 1 SE are indicated. The shaded area labeled "normal range" is the mean ± 1 SD 4CH 30 SB r 20- -r ;X 10- NORMAL RANGE T X , ALDOSTERONE (pg/24hr) FIGURE 2 Relationship between daily rate of aldosterone administration and plasma sodium concentration. Shaded areas represent the means and standard error of the measurements of each of the four levels of infusion. Data from the individual dogs are represented by the same symbol at each level of infusion. Measurements were made on each of the final 3 days of each infusion period. The curve through the data was derived by inspection. urinary excretion rate of a group of eight intact dogs eating the same sodium and potassium intake as the dogs in the present experiment. At each level of aldosterone infusion the average excretion rate is within or very close to the normal range, indicating the dogs were in a steady state of sodium and potassium balance. Differences between the rates of excretion at the different infusion levels probably are the results of errors in urine collection rather than actual differences in the steady state excretion rates. PLASMA SODIUM CONCENTRATION Figure 2 is a plot of the relationship between aldosterone levels and plasma sodium concentration. The normal level of plasma sodium concentration for dogs in our laboratory is between 140 and 143 meq/liter, a level that, according to the plot of Figure 2, could be achieved by infusion of between 48 and 80 pig/day of aldosterone. Aldosterone levels appeared to have a potent influence on plasma sodium concentration at the low levels of infusion but very little effect at the infusion rates above the normal range. EFFECTS OF ALDOSTERONE ON PLASMA POTASSIUM CONCENTRATION 40-, : :x UJ 20- NORMAL RANGE 10- ALDOSTERONE.... j (pg/24hr> I -i (16) (48) m (91) (219) FIGURE 1 Average daily urinary sodium (top) and potassium (bottom) excretion rates at the end of the four levels of aldosterone infusion. The shaded areas represent the normal range of excretion rates for intact dogs (n = 8) on the same level of electrolyte intake as the dogs in the study. Changes in aldosterone infusion rates produced very consistent changes in plasma potassium concentration, as can be seen in Figure 3. Again, the effect of the hormone was greatest at the lower levels of infusion. Normal plasma potassium concentration in the dogs in our laboratory is between 3.75 and 4.00 meq/liter. According to the plot of Figure 3, this level could be achieved by infusion of between 40 and 60 ^ig/day in dogs of this size on this particular diet. EFFECTS OF ALDOSTERONE ON PLASMA RENIN ACTIVITY Plasma renin activity of dogs in our laboratory eating a diet containing 27 meq/day each of sodium and potassium

3 140 CIRCULATION RESEARCH VOL. 40, No. 2, FEBRUARY 1977 loooon ALDOSTERONE(pg /24 hr) FIGURE 3 Relationship between daily rate of aldosterone administration and plasma potassium concentration. Symbols as in Figure 2. per day is normally between 0.7 and 1.5 ng/ml per hour. According to the plot of Figure 4, this level would be achieved by aldosterone infusions of approximately /ug/day. As one can see in Figure 4, plasma renin activity levels rise at a rapid rate as aldosterone levels fall below normal and, conversely, drop to zero at rates of aldosterone administration slightly above normal. This dramatic relationship between aldosterone and renin probably is involved in producing the complex relationship between aldosterone, on the one hand, and sodium space and mean arterial pressure, on the other hand, shown in Figures 5 and 6. SODIUM SPACE AND BODY WEIGHT Sodium space increased with increasing aldosterone at the two lower levels of aldosterone infusion, and then ALD0STER0NE(jjq/24hr) FIGURE 5 Relationship between rate of aldosterone administration and sodium space. Symbols as in Figure 2. appeared to plateau as the rate of infusion increased above the normal rate (Fig. 5). The mean value actually decreased slightly as the infusion rate was more than doubled from level III, 91 ^.g/24 hours, to level IV, 219 /xg/24 hours. Body weight increased progressively as the level of aldosterone infusion was increased. Going from level I to IV the weights (in kilograms) averaged ± 0.63, ± 0.62, ± 0.85, and ± MEAN ARTERIAL PRESSURE The relationship between aldosterone and arterial pressure (Fig. 6) was complex. Normal arterial pressure in conscious dogs in our laboratory averages about 100 mm Hg. Readings close to this level were found both at the second level of infusion, which is close to the normal aldosterone secretion rate, and at the first level of infusion, probably 'U to l /i the normal secretion rate. At these two lower rates of aldosterone administration there was no positive correlation between aldosterone and pressure. But as aldosterone infusion increased above the normal level, up to 91 and 219 /xg/24 hours, the pressure did rise, reaching a level of 119 ± 2 mm Hg at the highest aldosterone level a. a. < 100 E ALDOSTERONE ( uq /24 hr) FIGURE 4. Relationship between daily rate of aldosterone administration and plasma renin activity (PRA). Symbols as in Figure ALDOSTERONE (pg/24hr) FIGURE 6 Relationship between rate of aldosterone administration and mean arterial pressure (MAP). Symbols as in Figure 2.

4 ALDOSTERONE DOSE-RESPONSE/Young and Guyton 141 Discussion By infusing measured amounts of aldosterone into adrenalectomized dogs for several weeks at each of four rates, we were able to obtain steady state dose-response relationships between aldosterone and five variables. Plasma sodium concentration and sodium space both declined sharply as aldosterone infusion rates fell below normal, but these variables increased only slightly above normal as aldosterone infusion rates were increased to approximately 4 times normal. This type of relationship suggests that aldosterone is not the prime controller of sodium concentration but, rather, a minimum level of aldosterone must be maintained for other mechanisms to be effective in its actual control. This conclusion is supported by two studies recently completed in this laboratory. In the first, 6-7 feedback control of aldosterone secretion was blocked in dogs and then sodium intake was increased from a low level (10 meq/day) to a high level (200 meq/ day). Feedback control of aldosterone secretion was blocked by adrenalectomy several weeks prior to the study and then maintaining the days on a fixed amount of infused aldosterone (100 /xg/day) throughout both low and high sodium intake periods. In that study, the 20-fold increase in sodium intake resulted in a 2% change in plasma sodium concentration, exactly the same change seen when the same 20-fold increase in sodium intake was given to a group of intact dogs in which the level of aldosterone secretion could change in response to the change in intake. Although both the group without feedback control of aldosterone secretion and the intact group possessed the same ability to control plasma sodium concentration precisely, control of sodium balance was greatly impaired in the group lacking functional aldosterone control; in response to the increase in sodium intake the control group showed a 2% increase in 22 Na space, whereas the experimental group had a 12% increase in sodium space. From those data it was concluded that the aldosterone feedback control system is not important in control of extracellular sodium concentration although it is of significant importance in regulating sodium balance. A second group of experiments sought to determine how sodium concentration was controlled if aldosterone was not involved. In this study of degree of involvement of the antidiuretic hormone (ADH)-thirst feedback control system in regulating extracellular sodium concentration was assessed. 8 ' 9 The same 20-fold increase in sodium intake was again given, first to a group of intact dogs, then to a group with blocked feedback control of ADH secretion and water intake. Feedback control was blocked by continuously infusing ADH at a rate which maintained the dogs' urine osmolality at a maximal level (greater than 1,500 mosmol/liter) during the low and the high intake period, and by giving the dogs a mandatory water intake of 700 ml/day during both low and high intake periods. Again, the 20-fold increase in sodium intake resulted in a 2% increase in sodium concentration in the intact group, but now a 12% increase in sodium concentration was measured in the group with blocked feedback control of the ADH-thirst system. From this study it was concluded that the ADH-thirst feedback control system was the primary controller of extracellular sodium concentration. Although no ADH measurements were made during the study described in the present paper, it is reasonable to propose that the fall in sodium concentration observed at the lowest level of aldosterone infusion did result from a stimulation of the ADH system due to sodium depletion and hypovolemia. 10 This contention is supported by data from severe Addisonian patients, showing that such patients have high levels of ADH associated with hyponatremia and volume depletion." Plasma potassium concentration appears to be controlled mainly by the aldosterone feedback mecanism, as evidenced by the consistent changes in plasma potassium levels resulting from altering the aldosterone infusion rate from the lowest to the highest rate. The complex relationship between aldosterone and arterial pressure may be explained by the participation of angiotensin in control of arterial pressure and sodium and volume balance. At the normal aldosterone level (48 /xg/ 24 hours) fluid volume, plasma renin activity, and arterial pressure were at or near normal values. At the lower aldosterone level (16 /xg/24 hours) pressure was still normal while sodium space was decreased by an average of 952 ml, or 13%. It is possible that this large decrease in fluid volume did not cause a change in arterial pressure because of the concomitant very marked rise in renin activity levels. The difference in plasma renin activity between the two levels was 457%, 9.6 vs. 2.1 ng/ml per hour. Under several conditions of reduced volume, such as sodium depletion 12 '' 3 and hemorrhage, increased renin secretion has been shown to be a major factor in maintaining normal arterial pressure. Therefore, it is reasonable to suggest that the pressor action of the high level of angiotensin may have been responsible in the experiments for maintaining arterial pressure during the reduced volume state. As aldosterone levels were increased above the normal level and arterial pressure began to rise, plasma renin activity fell abruptly to zero. With renin levels maximally suppressed, further increases in fluid volume could no longer be compensated for by further reducing the pressor contribution of angiotensin; this possibly explains the rise in arterial pressure when aldosterone levels and fluid volume were increased above normal even though the measured fluid volume increase was minimal. The highest average arterial pressure recorded in this study was 119 ± 2 mm Hg. This pressure was the steady state result of aldosterone infused at rates 4'/2 times normal. In this experimental situation it would not have been possible to increase arterial pressure much higher by giving additional aldosterone, since pilot studies showed that slightly greater mineralocorticoid activity would lead to debilitating or fatal potassium depletion. In the pilot experiment, two adrenalectomized dogs eating the same diet used in the present study were given /xg/day of aldosterone for 2 weeks, after which the study had to be discontinued because the dogs' plasma potassium levels had dropped below 2.00 meq/liter. Therefore, unless potassium intake is radically increased, the effects of mineralocorticoids alone probably cannot raise arterial pressure more than about 30 mm Hg in the dog. If these results

5 142 CIRCULATION RESEARCH VOL. 40, No. 2, FEBRUARY 1977 have relevance to human physiology, they also suggest that the very high arterial pressures occasionally seen in primary aldosteronism may be only partially due to the effects of the excessive mineralocorticoid levels. An important additional factor contributing to the marked hypertension in these cases may be vascular lesions, especially in the renal preglomerular bed, 16 resulting from prolonged increased pressure, particularly in view of the fact that the arterial pressure often fails to return to normal following removal of the causative adrenal tissue." ia These results also suggest that the renin-angiotensin system may have a direct effect on sodium balance control. The dogs were in a steady state of sodium balance at infusion levels I and II even though at level I only 33% as much aldosterone was being given as at level II. Apparently the natriuretic effect of the reduction in aldosterone administration was partially balanced by an increase in the level of other antinatriuretic agents or factors. Probably one factor involved was angiotensin II, because in acute experiments in several laboratories, intravenous renal and arterial angiotensin II infusions at high physiological rates have produced a fall in sodium excretion. 19 " 23 Therefore, it is likely that the 472-fold increase in plasma renin activity (and presumed resultant increase in angiotensin II) accounted for a portion of the compensatory sodium reabsorption in the present experiment. Apparently as well as affecting sodium balance indirectly through its stimulation of aldosterone, the renin-angiotensin system also plays an important parallel role in sodium control by directly affecting sodium reabsorption by the kidney. In summary, the results of this study describe the quantitative steady state relationships between aldosterone and plasma sodium concentration, potassium concentration, plasma renin activity, sodium space, and arterial pressure. Decreasing aldosterone levels below normal strongly affects plasma sodium concentration and sodium space, whereas raising aldosterone above normal has little effect on these variables. Plasma potassium concentration varies in a highly consistent manner with changes in aldosterone levels. Plasma renin activity rises rapidly as aldosterone falls below normal and drops to zero at aldosterone levels slightly above normal. Arterial pressure increases as aldosterone rises above normal, but does not fall below normal at subnormal aldosterone levels, probably because of the pressor effects of angiotensin II. References 1. Cowley AW Jr, Liard JF, Guyton AC: Role of baroreceptor reflex in daily control of arterial blood pressure and other variables in dogs. Circ Res 32: , Haber E, Koerner T, Page LB, Kliman B, Purnode A: Application of a radioimmunoassay for angiotensin I to the physiological measurements of plasma renin activity in normal human subjects. J Clin Endocrinol Metab 29: , Liard JF, Cowley AW Jr, McCaa RE, McCaa CS, Guyton AC: Renin, aldosterone, body fluid volumes and the baroreceptor reflex in the development and reversal of Goldblatt hypertension in conscious dogs. Circ Res 34: , Norman RA Jr, Coleman TG, Wiley TL Jr, Manning RD Jr, Guyton AC: Separate roles of sodium ion concentration and fluid volumes in salt-loading hypertension in sheep. Am J Physiol 229: , Manning RD Jr: Hemodynamic and humoral changes during the initial phases of salt-induced renoprival hypertension. Ph.D. dissertation, University of Mississippi, Young DB, Guyton AC: The role of the aldosterone system in control of fluid volume and plasma electrolytes. Fed Proc 33: 340, Young DB, McCaa RE, Pan YJ, Guyton AC: Effectiveness of the aldosterone-sodium and potassium feedback control system. Am J Physiol 231: , Young DB, Guyton AC: Control of plasma sodium concentration by the ADH-thirst mechanism. Physiologist 17: 364, Young DB, Pan YJ, Guyton AC: Control of extracellular sodium concentration by the ADH-thirst feedback mechanism. Am J Physiol (accepted for publication) 10. Hays RM, Levine SO: Pathophysiology of water metabolism. In The Kidney, edited by BM Brenner, FC Rector Jr. Philadelphia, Saunders, 1976, pp Ahmed ABJ, George BC, Gonzalez-Aubert C, Dingman JF: Increased arginine vasopressin in clinical adrenocortical insufficiency and its inhibition by glucosteroids. J Clin Invest 46: , Coleman TG, Cowley AW Jr, Guyton AC: Angiotensin and the hemodynamics of chronic salt deprivation. Am J Physiol 229: , Gavras H, Brunner HR, Vaughn ED Jr, Laragh JH: Angiotensinsodium interaction in blood pressure maintenance in renal hypertensive and normotensive rats. Science 180: , Erdos EG, Massion WH, Downs DR, Geese A: Effect of the inhibition of angiotensin I converting enzyme in endotoxin and hemorrhagic shock. Proc Soc Exp Biol Med 145: , Freeman RH, Davis JO, Johnson JA, Spielman WS, Zatsman ML: Arterial pressure regulation during hemorrhage; hemostatic role of angiotensin II. Proc Soc Exp Biol Med 149: 19-22, Guyton AC, Cowley AW Jr, Coleman TG, Liard JF, McCaa RE, Manning RD Jr, Norman RA Jr, Young DB: Pretubular versus tubular mechanisms of renal hypertension. In Mechanisms of Hypertension, edited by MB Sambhi. New York, Excerpta Medica, 1973, pp Brown JJ, Davies DL, Ferriss JB, Fraser R, Haywood E, Lever AF, Robertson JIS: Comparison of surgery and prolonged spironolactone therapy in patients with hypertension, aldosterone excess and low plasma renin. Br Med J 2: 729, Biglieri EG, Schambelan M, Slaton PE, Stockigt JR: The intercurrent hypertension of primary aldosteronism. Circ Res 27 (suppl I): , Waugh WH: Angiotensin II; local renal effects of physiological increments in concentration. Can J Physiol Pharmacol 50: , Navar LG, Langford HG: Effects of angiotensin on the renal circulation. In Handbook of Experimental Pharmacology, vol 37, edited by IH Page, FM Bumpus. New York, Springer-Verlag, 1973, pp Johannesen J, Omvik JR, Kiil F: Hemodynamic mechanisms influencing sodium excretion during angiotensin infusion. Am J Physiol 230: , DeClue JW, Coleman TG, Cowley AW Jr, McCaa RE, Guyton AC: Influence of angiotensin II (AN) on the long-term renal excretion of sodium. Fed Proc 35: 397, DeClue JW, Cowley AW Jr, Coleman TG, McCaa RE, Guyton AC: Influence of long-term infusions of angiotensin II on arterial pressure, plasma aldosterone concentration and renal sodium excretion. Physiologist 19: 165, 1976