Expression and function of a T-type Ca~(2+) conductance in interstitial cells of Cajal of the murine small intestine

摘要

Interstitial cells of Cajal (ICC) generate slow waves in gastrointestinal (GI) muscles. Previous studies have suggested that slow wave generation and propagation depends on a voltage-dependent Ca~(2+) entry mechanism with the signature of a T-type Ca~(2+) conductance. We studied voltage-dependent inward currents in isolated ICC. ICC displayed two phases of inward current upon depolarization: a low voltage-activated inward current and a high voltage-activated current. The latter was of smaller current density and blocked by nicardipine. Ni~(2+) (30 muM) or mibefradil (1 muM) blocked the low voltage-activated current. Replacement of extracellular Ca~(2+) with Ba~(2+) did not affect the current, suggesting that either charge carrier was equally permeable. Half-activation and half-inactivation occurred at -36 and -59 mV, respectively. Temperature sensitivity of the Ca~(2+) current was also characterized. Increasing temperature (20-30°C) augmented peak current from -7 to -19 pA and decreased the activation time from 20.6 to 7.5 ms [temperature coefficient (Q_(10)) = 3.0]. Molecular studies showed expression of Cacnalg (Cav3.1) and Cacnalh (Cav3.2) in ICC. The temperature dependence of slow waves in intact jejunal muscles of wild-type and Cacnalh~(-/-) mice was tested. Reducing temperature decreased the upstroke velocity significantly. Upstroke velocity was also reduced in muscles of Cacnalh~(-/-) mice, and Ni~(2+) or reduced temperature had little effect on these muscles. Our data show that a T-type conductance is expressed and functional in ICC. With previous studies our data suggest that T-type current is required for entrainment of pacemaker activity within ICC and for active propagation of slow waves in ICC networks.