EFFECT OF LITHIUM AND ANTIDIURETIC HORMONE ON PLASMA RENIN CONCENTRATION IN DIABETES

INSIPIDUS RATS (BRATTLEBORO RAT MODEL)

S. Opava-Stitzer

Department of Physiology University of Puerto Rico School of Medicine

San Juan, Pue,.to Rico 00936

Antidiuretic hormone (ADH) is known to inhibit renin secretion in many species,l-G but the mechanism of this inhibition and its importance in the control of renin secretion are unknown. Measurements of plasma renin concentration (PRC) and plasma renin activity (PRA) in the diabetes insipidus (D!) rat suggest that physiological levels of ADH may exert a tonic inhibitory effect on renin release. PRC and PRA are increased in the D1 rat 7-9 and reduced follow­ing treatment with ADH.9-11 Nevertheless it is unclear to what extent these alterations in renin are due to the presence or absence of ADH per se. In the absence of ADH, plasma volume,12 blood pressure,7 and glomerular filtration rate 19 are reduced, all of which might stimulate renin secretion. Conversely, ADH might reduce plasma renin levels in the Dr rat indirectly, secondary to its effects on water balance, as well as by a direct effect on renin secretion. To differentiate between direct and indirect effects of ADH on renin secretion, plasma renin concentration was measured in Dr rats, before and during ADH administration, with or without LiCl treatment. Using different doses of ADH, combined with LiCI treatment, it was possible to raise the titer of antidiuretic hormone in the plasma of the Dr rat while blocking to a varying extent the action of ADH on tubular water reabsorption. l1 The effect of ADH on PRe of LiCI-treated and control (NaCI-treated) Dl rats was compared.

All rats were maintained in individual metabolism cages for the entire study. As shown in TABLE 1, the protocol consisted of pretreatment with 0.1 ml/lOO g BW/day of isotonic saline (N= 37) or LiCI (3.0 mEq/kg BW/day) (N=38), Lp., for 11 days, followed by two weeks of balance study and bleeding for determination of PRC. Forty-three rats (half LiCI-, half NaCl-treated) were then injected with 10 mU (N= 12),25 mU (N= 11), or 200 mU (N=20) of ADH (Parke-Davis, Pitressin tannate in oil) per 100 g BW per day, or an equal volume of sesame oil (N =22), s.c., for one week, followed by two weeks of balance study and bleeding. Once begun, LiCI, NaCI, ADH, and oil injections were continued throughout the study. Ten rats received neither ADH nor oil but only LiCI (N = 5) or NaCI (N = 5) injections. Blood samples were obtained by tailcutting in the conscious rat. PRC was determined by radioimmunoassay (New England Nuclear) 15 of the amount of angiotensin I generated in the presence of excess substrate.16 Balance studies consisted of daily measurement of body weight, water intake, urine volume, osmolality, and sodium and potassium concentrations. The effects of an intermediate dose of ADH (20 mU/100 g BW/day) on hematocrit, plasma osmolality, plasma protein, and sodium and potassium concentrations were also determined in 10 LiCl-treated and 10 NaCI-treated Dr rats. Sodium and potassium concentra­

TABLE 1

PROTOCOL FOR TIlE STUDY OF TIIE EFFECTS OF ADH'TREATMENT ON PLASMA RENIN

CONCENTRATION IN LrCL-TREATED AND CONTROL (NACL-TREATED) DI RATS

Time (days)

1-11 12-18 19-25 26-32 33-39 40--44

Procedures 1. NaCl or LiCl 1. NaCl or LiCl 1. NaCl or LiCl 1. NaClor LiCl 1. NaCl or LiCl 1. NaClor LiCl injections injections injections injections injections injections

2. 2-Day balance 2. 2-Day balance 2. ADH or oil 2. ADH or oil 2. ADH or oil study study injections injections injections (Days 13-15) (Days 20-22) 3. 2-Day balance 3. 2-Day balance

3. Renin 3. Renin study study (Day 16) (Day 23) (Days 34-36) (Days 41--43)

4. Renin 4. Renin (Day 37) (Day 44)

tions in urine and plasma were determined by flame photometry; osmolality by freezing-point depression.

The effect of ADH on urine osmolality was greater during the second week of study and only these results have been included in FIGURES I and 2. As shown in FIGURE I, ADH had widely differing effects on urine osmolality of LiCI- and NaCI-treated rats. At doses of 10 and 25 mU/lOO g BW/day, LiCI-treated rats continued to excrete hypotonic urine while the urine of NaCI­treated rats was hypertonic. At the highest dose of ADH both groups were able to concentrate their urine, although to differing degrees. Despite these disparate effects on water excretion, ADH affected PRC similarly in both groups. As shown in FIGURE 2, at any dose of ADH, there was no difference in the PRC of LiCI- and NaCI-treated 01 rats. These data also suggest that a dose-response relationship may have existed between ADH and renin secretion with maximal effects obtained at 25 mU /1 00 g BWI day. There was no difference in the effect of this dose and the 200 mD dose on PRe. Although the 10 mD dose of ADH

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FIGURE 1. Effect of different doses of ADH or oil (ADH =0) on urine osmolality (Uo. rn ) of LiCl-treated and NaCl-treated DI rats. *Significantly different from values prior to ADH or oil treatment, p < 0.05. +Significantly different from NaCl­treated rats, p < 0.05. Values are means ± SEM.

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did not alter PRC, oil injections alone (ADH = 0, FIGURE 2) increased PRe, as did multiple bleedings of LiCl- and NaCl-treated rats with neither ADH nor oil injections superimposed. These results suggest that the lowest dose of ADH may have prevented such a rise in PRC.

As shown in FIGURE 3, when all data were pooled, there was no significant correlation between plasma renin concentration and urine osmolality. More­over, there was no difference in PRC when the urine was hypertonic (78 ± 4 ng AI/ml/h, N = 65) or hypotonic (82 ± 5, N = 84).

As shown in TABLE 2, ADH treatment significantly affected only plasma osmolality, which was decreased by ADH in control (NaCl) rats, but increased in LiCI-treated rats, probably not as a result of, but despite ADH treatment.

These data indicate that the inhibition of renin secretion in the DI rat during ADH treatment is unrelated to effects of ADH on urine concentration. Thus systemic effects on renin release, secondary to. changes in water balance, seem an unlikely explanation for the observed inhibition of renin. Rather a direct action of ADH on the renin-producing cell or baroreceptor is suggested.

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FIGURE 3. Linear regression analysis of the relationship between plasma renin concentration and urine osmolality in LiCl-treated and NaCl-treated Dr rats. The solid line is the regression line for values during ADH treatment only; the dotted line is the regression line for all values.

TABLE 2

EFFECTS OF ADH TREATMENT ON HEMATOCRIT (HCf), PLASMA OSMOLALITY (POsm), AND PLASMA PROTEIN, SODIUM (PN.) AND POTASSIUM (px) CONCENTRATION

IN LiCl- AND NaCl-TREATED Dr RATS

HCT Protein Posm PNs Px (%) (g %) (mOsmlL) (mM) (mM)

LiCl Control 39 ± 0.6 7.5 ± 0.17 294 ± 4 142 ± 0.7 3.6 ± 0.09

ADH 40' ± 0.7 7.4 ± 0.09 305 ± 3 t 143 ± 0.8 3.6 ± 0.11

NaCl Control 39 ± 0.9 7.7 ±'0.12 313 ± 2 142 ± 1.3 3.4 ± 0.11

ADH 41 ± 0.7 7.8 ± 0.17 308 ± 1 * 142 ± 1.4 3.1 ± 0.12

Values are means ± SEM. * Significantly different from control, P < 0.05; t P < 0.001.

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