Astract-Preglomerular responses to vasoactive agonists utilize calcium released from intracellular stores and activation of calcium influx pathways to elicit vasoconstriction. The current study was performed to determine the role of calcium release from intracellular stores on the afferent arteriolar response to increases in perfusion pressure. Experiments were performed, in vitro, using the blood perfused, juxtamedullary nephron technique combined with videomicroscopy. The response of afferent arterioles to 30 mm Hg increases in perfusion pressure was determined before and after depletion of intracellular calcium pools with a 10-minute preincubation with 1 micro mol/L thapsigargin or 100 micro mol/L cyclopiazonic acid. Afferent arteriolar diameter averaged 20.2 +/-1.0 micro meter (n = 19) at a control perfusion pressure of 100 mm Hg. Increasing perfusion pressure to 130 and 160 mm Hg reduced afferent caliber by 10.7 +/- 1.0% (P <.05 versus con) and by 24.7 +/- 1.6% (P <.05 versus diameter at 130 mm Hg); respectively. Thapsigargin significantly increased afferent diameter by 21 +/- 2% (n = 6) at 100 mm Hg and prevented pressure-induced autoregulatory responses. Afferent diameter averaged 24.3 +/- 1.7, 24.5 +/- 1.8 and 24.3 +/- 1.8 micro meter at perfusion pressures of 100, 130 and 160 mm Hg; respectively. Cyclopiazonic acid treatment also inhibited autoregulatory behavior but did not alter resting vessel diameter. Afferent arteriolar diameter (n = 6) averaged 21.4 +/- 1.9 micro meter at 100 mm Hg and 20.9 +/- 2.1 and 20.5 +/- 2.2 micro meter at 130 and 160 mm Hg; respectively. Additional studies were performed to assess the role of phospholipase C activity in pressure-mediated autoregulatory behavior of afferent arterioles. Step increases in perfusion pressure decreased afferent diameter by 10.7 +/- 3.8 and 21.7 +/- 4.1%; respectively. Administration of the phospholipase C inhibitor, U-73122, (5 micro moles/L) did not significantly alter baseline diameter but did attenuate the pressure-mediated vasoconstrictor response. Increasing perfusion pressure to 130 and 160 mm Hg reduced afferent diameter by only 6.5 +/- 1.5 and 10.0 +/- 2.0%; respectively. These data demonstrate that interruption of calcium mobilization with thapsigargin, cyclopiazonic acid, or phospholipase C inhibition markedly attenuates pressure-mediated afferent arteriolar vasoconstriction and suggests that autoregulatory adjustments in afferent arteriolar diameter involve calcium release from inositoltrisphosphate (IP3)-sensitive intracellular stores. (Hypertension. 1998;31[part 2]:421-428.). © 1998 Lippincott Williams & Wilkins, Inc.