RENOVASCULAR RESPONSES TO HIGH AND LOW PERFUSATE CALCIUM STEADY-STATEEXPERIMENTS IN THE ISOLATED-PERFUSED RAT-KIDNEY WITH BASE-LINE VASCULAR TONE

Acute hypercalcemia is commonly observed in surgical patients after ca lcium infusion while acute hypocalcemia is common during rapid citrate d blood transfusion. Although high and low ionized calcium ([Ca2+]) wi thin the clinical range produce an increase or decrease in cardiac per formance and systemic vessel resistance, respectively, their effects o n renal vessels have not been quantified. A possible renal vasoconstri ction that might occur with high [Ca2+] is of clinical interest becaus e it is a factor which may contribute to impaired renal circulation an d decreased function, In this study we examined the renovascular respo nses to [Ca2+], which was varied within the clinical range under hemod ynamically controlled conditions. We instituted high and low [Ca2+] in the per fusate, which consisted of Krebs-Henseleit buffer containing albumin, 60-65 g/liter. Stable high (n = 10) or low (n = 7) [Ca2+] (1. 93 +/- 0.02 and 0.59 +/- 0.01 mM, respectively) was instituted for 10 min and preceded and followed by normal [Ca2+] of the same duration. I n a separate protocol (n = 8) verapamil (10(-5) M) was added to the pe rfusate 10 min before high [Ca2+] was tested. We measured changes in r enal flow at a constant perfusion pressure of 110 mm Hg and also chara cterized the renal vessels over a range of pressures by pressure vs fl ow plots. High [Ca2+] was associated with a small decrease in how (fro m 28.8 +/- 2.4 to 26.9 +/- 2.6 ml/min/g, P < 0.02), indicating a small vasopressor effect. This effect was also shown by a leftward shift in the pressure vs flow plots, These changes were prevented by verapamil . GFR decreased (from 0.35 +/- 0.04 to 0.28 +/- 0.06 ml/min/g, P < 0.0 1) without a significant change in sodium excretion or fractional sodi um excretion. Low [Ca2+] was associated with increased renal flow (fro m 30.8 +/- 2.1 to 35.2 +/- 2.7 ml/min/g, P < 0.02), indicating a vasod ilator effect. This effect was also shown by a rightward displacement of the pressure vs how plots. GFR increased from 0.51 +/- 0.03 to 0.56 +/- 0.04 ml/min/g, P < 0.01, as did sodium excretion (from 2.32 +/- 0 .22 to 3.87 +/- 0.49 mu Eq/min, P < 0.01) and fractional sodium excret ion (from 2.33 +/- 0.26 to 3.61 +/- 0.49%, P < 0.01), We conclude, fir st, that in the isolated perfused rat kidney, high [Ca2+] is a weak va sopressor while low [Ca2+] has vasodilator action. Second, high [Ca2+] effects are abolished by verapamil pretreatment. These findings illum inate mechanisms of high [Ca2+] effects on renovascular tone. (C) 1996 Academic Press, Inc.