Two types of Ca(2+)-channels in cells from the circular layer of guinea-pig ileum

In this paper data are presented concerning some morphometrical characteristics and passive electrical properties of cell membranes from the circular layer of guinea-pig ileum (CC). The main purpose of the study has been to analyze the inward calcium current in these cells. Using the whole-cell voltage-clamp method the presence of T-like type of calcium channels (conducting low-threshold, fast inactivated, strictly potential-dependent and dihydropyridine-resistant calcium inward current--If) could be found in membranes of the cells investigated. The participation of If in the total inward current of CC was deduced from the presence of a low-threshold "hump" in the current-voltage relation curve (I/V-curve) between -80 and -50 mV, and also from a negative shift of this curve at holding potential Vh = -90 mV, compared to the one obtained at Vh = -50 mV. Besides, in the presence of a dihydropyridine Ca2+ antagonist, a fast inactivating component of the inward current could be recorded, which was effectively blocked by 0.3 mmol/l Ni2+ from outside. According to Tsien's criteria (1983) our data suggest the coexistence of T- and L-types calcium channels in CC.