Contractility of vascular smooth muscle cells (VSMCs) depends on a rise of cytosolic [Ca²⁺] resulting from two major processes: (i) Ca²⁺ influx through voltage-dependent Ca²⁺ channels (VDCCs) of the plasma membrane or (ii) Ca²⁺ release from the sarcoplasmic reticulum (SR). Although it has been suggested that agonist-induced Ca²⁺ release from internal stores can be modulated by membrane voltage (e.g. Ganitkevich & Isenberg, 1993), the underlying mechanisms are unknown. We have investigated whether depolarization can induce Ca²⁺ release from internal stores in myocytes freshly dispersed from rat basilar artery using patch-clamp and microfluorimetry.

Animals were killed by I.P. injection of pentobarbital (50 mg kg^-1). Activation of VDCCs by either direct membrane depolarization in voltage-clamped myocytes or application of solutions with 70 mM K⁺ in undialysed cells evoked sharp, transient [Ca²⁺]_i release signals in the absence of extracellular Ca²⁺ plus 2 mM EGTA. This response was suppressed after inhibition of the Ca²⁺-ATPase by thapsigargin, although it was present when actin cytoskeleton desorganization was induced with calyculin A and jasplakinolide (Patterson et al. 1999). Inhibition of G protein, phospholipase C and InsP₃ receptor with GDPβS, U73122 and 2-amino-ethoxydiphenyl borate, respectively, abolished the increase in [Ca²⁺]_i. These results suggested that depolarization-induced Ca²⁺ release (DICR) operates via G protein activation and InsP₃ production. In addition, several lines of evidence indicated that activation of G protein by membrane depolarization depends on VDCCs. (i) The DICR vs. voltage relationship was similar to the conductance-voltage curve for L-type VDCCs; (ii) Ca²⁺ release was drastically reduced when instead of applying depolarizing voltage steps, the membrane potential was changed slowly, leading to inactivation of the channels; and (iii) DICR was potentiated by agonists (Bay K 8644 and FPL 64176) and inhibited by antagonists (diltiazem) of VDCCs. These unprecedented observations suggest a novel role of VDCCs inVSMCs. Besides mediating transmembrane Ca²⁺ influx, Ca²⁺ channels act as voltage sensors that on cell depolarization induce InsP₃ production and Ca²⁺ release from SR to further increase cytosolic [Ca²⁺].

This work was supported by a grant from the Spanish Ministry of Science and Education (PM1999-0120) and to the ‘Ayuda a la Investigación 2000’ of the Juan March Foundation. A. del V. is a fellow of FIS.