R-type calcium channels in myenteric neurons of guinea pig small intestine

Currents carried by L-, N-, and P/Q-type calcium channels do not account for the total calcium current in myenteric neurons. This study identified all calcium channels expressed by guinea pig small intestinal myenteric neurons maintained in primary culture. Calcium currents were recorded using whole cell techniques. Depolarizations ( holding potential = - 70 mV) elicited inward currents that were blocked by CdCl2 ( 100 muM). Combined application of nifedipine ( blocks L- type channels), Omega-conotoxin GVIA ( blocks N- type channels), and Omega-agatoxin IVA ( blocks P/Q-type channels) inhibited calcium currents by 56%. Subsequent addition of the R-type calcium channel antagonists, NiCl2 ( 50 muM) or SNX-482 ( 0.1 muM), abolished the residual calcium current. NiCl2 or SNX-482 alone inhibited calcium currents by 46%. The activation threshold for R-type calcium currents was - 30 mV, the half-activation voltage was - 5.2 +/- 5 mV, and the voltage sensitivity was 17 +/- 3 mV. R-type currents activated fully in 10 ms at 10 mV. R-type calcium currents inactivated in 1 s at 10 mV, and they inactivated ( voltage sensitivity of 16 +/- 1 mV) with a half-inactivation voltage of - 76 +/- 5 mV. These studies have accounted for all of the calcium channels in myenteric neurons. The data indicate that R-type calcium channels make the largest contribution to the total calcium current in myenteric neurons. The relatively positive half-activation voltage and rapid activation kinetics suggest that R-type channels could contribute to calcium entry during somal action potentials or during action potential-induced neurotransmitter release.