Contribution of transient and sustained calcium influx, and sensitization to depolarization-induced contractions of the intact mouse aorta.

Background: Electrophysiological studies of L-type Ca2+ channels in isolated vascular smooth muscle cells revealed that depolarization of these cells evoked a transient and a time-independent Ca2+ current. The sustained, non-inactivating current occurred at voltages where voltage-dependent activation and inactivation overlapped (voltage window) and its contribution to basal tone or active tension in larger multicellular blood vessel preparations is unknown at present. This study investigated whether window Ca2+ influx affects isometric contraction of multicellular C57Bl6 mouse aortic segments.

Results: Intracellular Ca2+ (Cai2+, Fura-2), membrane potential and isometric force were measured in aortic segments, which were clamped at fixed membrane potentials by increasing extracellular K+ concentrations. K+ above 20 mM evoked biphasic contractions, which were not affected by inhibition of IP3- or Ca2+ induced Ca2+ release with 2-aminoethoxydiphenyl borate or ryanodine, respectively, ruling out the contribution of intracellular Ca2+ release. The fast force component paralleled Cai2+ increase, but the slow contraction coincided with Cai2+ decrease. In the absence of extracellular Ca2+, basal tension and Cai2+ declined, and depolarization failed to evoke Cai2+ signals or contraction. Subsequent re-introduction of external Ca2+ elicited only slow contractions, which were now matched by Cai2+ increase. After Cai2+ attained steady-state, isometric force kept increasing due to Ca2+- sensitization of the contractile elements. The slow force responses displayed a bell-shaped voltage-dependence, were suppressed by hyperpolarization with levcromakalim, and enhanced by an agonist of L-type Ca2+ channels (BAY K8644).

Conclusion: The isometric response of mouse aortic segments to depolarization consists of a fast, transient contraction paralleled by a transient Ca2+ influx via Ca2+ channels which completely inactivate. Ca2+ channels, which did not completely inactivate during the depolarization, initiated a second, sustained phase of contraction, which was matched by a sustained non-inactivating window Ca2+ influx. Together with sensitization, this window L-type Ca2+ influx is a major determinant of basal and active tension of mouse aortic smooth muscle.