Expression and function of a T-type Ca2+ conductance in interstitial cells of Cajal of the murine small intestine

Abstract

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 Ca2+ entry mechanism with the signature of a T-type Ca2+ 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. Ni2+ (30 μM) or mibefradil (1 μM) blocked the low voltage-activated current. Replacement of extracellular Ca2+ with Ba2+ 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 Ca2+ 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 (Q10) = 3.0]. Molecular studies showed expression of Cacna1g (Cav3.1) and Cacna1h (Cav3.2) in ICC. The temperature dependence of slow waves in intact jejunal muscles of wild-type and Cacna1h−/− mice was tested. Reducing temperature decreased the upstroke velocity significantly. Upstroke velocity was also reduced in muscles of Cacna1h−/− mice, and Ni2+ 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.