3 September 2012
Electrophysiological studies of L-type Ca 2+ channels in isolated vascular smooth muscle cells revealed that depolarization of these cells evoked a transient and a time-independent Ca 2+ 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 Ca 2+ influx affects isometric contraction of multicellular C57Bl6 mouse aortic segments.
Intracellular Ca 2+ (Ca i 2+, 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 IP 3- or Ca 2+ induced Ca 2+ release with 2-aminoethoxydiphenyl borate or ryanodine, respectively, ruling out the contribution of intracellular Ca 2+ release. The fast force component paralleled Ca i 2+ increase, but the slow contraction coincided with Ca i 2+ decrease. In the absence of extracellular Ca 2+, basal tension and Ca i 2+ declined, and depolarization failed to evoke Ca i 2+ signals or contraction. Subsequent re-introduction of external Ca 2+ elicited only slow contractions, which were now matched by Ca i 2+ increase. After Ca i 2+ attained steady-state, isometric force kept increasing due to Ca 2+- 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 Ca 2+ channels (BAY K8644).
The isometric response of mouse aortic segments to depolarization consists of a fast, transient contraction paralleled by a transient Ca 2+ influx via Ca 2+ channels which completely inactivate. Ca 2+ 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 Ca 2+ influx. Together with sensitization, this window L-type Ca 2+ influx is a major determinant of basal and active tension of mouse aortic smooth muscle.