Resistance to stretch, [Ca2+]i, and activation of swine arterial smooth muscle.

Rapid stretching of smooth muscle induces a large increase in stress (resistance to stretch), and stress gradually decreases to intermediate values (stress relaxation). This study elucidated whether [Ca2+]-dependent crossbridge activation or passive mechanical structures were responsible for resistance to stretch and stress relaxation in swine carotid media, a tissue that does not exhibit a myogenic response. Tissues were equilibrated at the optimal length for stress development (L0) and loaded with aequorin to estimate myoplasmic [Ca2+]. In tissues activated with contractile agonists, both resistance to stretch and the rate of stress relaxation appeared to correlate best with the stress present before stretch. Stretch-induced [Ca2+] transients had no major role in determining resistance to stretch or stress relaxation. In the unstimulated swine carotid, resistance to stretch was only slightly reduced and stress relaxation not affected by removal of extracellular Ca2+, suggesting that resistance to stretch and the rate of stress relaxation in unstimulated tissues was predominantly dependent on the passive components of smooth muscle rather than attached, Ca(2+)-dependent crossbridges. Incubation of tissues with tetraethylammonium ion had no measurable effect on stretch-induced [Ca2+] transients but increased resistance to stretch, suggesting that stretch-induced myogenic contractions may be mediated by an increase in the sensitivity of the contractile apparatus to [Ca2+]. Despite the ability of stretch to produce substantial increases in myoplasmic [Ca2+], the contractile apparatus of swine carotid is quite insensitive to the stretch-induced [Ca2+] elevations.