Long-term regulation of contractility and calcium current in smooth muscle

Longitudinal smooth muscle strips from guinea pig ileum were cultured in vitro for 5 days, and the relationship between extracellular Ca2+ and force in high-K+ medium was evaluated. In strips cultured with 10% fetal calf serum (FCS), this relationship was shifted to the right (50% effective concentration changed by 2-3 mM) compared with strips cultured without FCS. The shift was prevented by inclusion of verapamil (1 mu M) during culture and mimicked by ionomycin in the absence of FCS. The intracellular Ca2+ concentration ([Ca2+](i)) during stimulation with high-K+ solution or carbachol was reduced after culture with FCS, whereas the [Ca2+](i)-force relationship was unaffected. Cells were isolated from cultured strips, and whole cell voltage-clamp experiments were performed. Maximum inward Ca2+ current (10 mM Ba2+), normalized to cell capacitance, was almost three times smaller in cells isolated from strips cultured with FCS. Culture with 1 mu M verapamil prevented this reduction. These results suggest that increased [Ca2+](i) during culture downregulates Ca2+ current density, with associated effects on contractility.