Capacitative calcium entry in smooth muscle cells from preglomerular vessels

Calcium entry via voltage-gated L-type channels is responsible for at least half of the increase in cytosolic calcium ([Ca2+](i)) in afferent arterioles following agonist stimulation. re sought the presence of capacitative calcium entry in fresh vascular smooth muscle cells (VSMC) derived from rat preglomerular vessels. [Ca2+](i) was measured using fura-2 ratiometric fluorescence. Vasopressin V1 receptor agonist (V1R) (10(-7) M) increased [Ca2+](i) by similar to 100 nM. A calcium channel blocker (CCB), nifedipine or verapamil (10-7 M), inhibited the response by similar to 50%. V1R in the presence of CCB increased [Ca2+](i) from 106 to 176 nM, confirming that calcium mobilization and/or entry may occur independent of voltage-gated channels. In nominally Ca2+-free buffer, V1R increased [Ca2+](i) from 94 to 129 nM, denoting mobilization; addition of CaCl2 (1 mM) further elevated [Ca2+](i) to 176 nM, indicating a secondary phase of Ca2+ entry. Similar responses were obtained when CCB was present in calcium-free buffer or when EGTA was present. In nominally Ca2+-free medium, the sarcoplasmic reticulum Ca2+-ATPase inhibitors (SRCAI), thapsigargin and cyclopiazonic acid (CPA), increased [Ca2+](i) from 97 to 128 and 143 nM, respectively, and to 214 and 220 nM, respectively, when 1 mM extracellular Ca2+ was added. In the presence of verapamil, the results with CPA acid were nearly identical. In Ca2+-free buffer, the stimulatory effect of V1R or SRCAI on the Ca2+/fura signal was quenched by the addition of Mn2+ (1 mM), demonstrating divalent cation entry. These studies provide evidence for capacitative (store-operated) calcium entry in VSMC freshly isolated from rat preglomerular arterioles.