CA2-MUSCLE CELLS( CURRENTS IN HUMAN COLONIC SMOOTH)

Voltage-gated Ca2+ currents were investigated in single smooth muscle cells freshly isolated from the circular layer of the human colon (asc ending and descending portions) using the whole cell voltage-clamp tec hnique. Tissue samples were obtained at the time of therapeutic surger y. In physiological salt solution (containing 2 mM Ca2+), an inward cu rrent was observed when the cell membrane was depolarized in the prese nce of tetrodotoxin. This current disappeared when Ca2+ was removed fr om the bath solution and was inhibited when Ca2+ channel blockers were applied, indicating that the inward current was a Ca2+ current (I-Ca) . Changing the holding potential (HP) from -100 mV to more positive po tentials (e.g., -60 and -40 mV) markedly decreased the amplitude of I- Ca. The voltage dependence of steady-state activation and inactivation was represented by Boltzmann distributions; there was a substantial a mount of overlap (window current) between -60 and -10 mV. A fast-inact ivating I-Ca component followed by a slow-inactivating I-Ca component was observed in some cells from both ascending and descending colons. The fast I-Ca component was observed only when cells were held at -80 or -100 mV, and had a more negative threshold potential (-70 to -60 mV ). This component was sensitive to low concentrations of Ni2+ (30 mu M ) but was resistant to nifedipine (10-20 mu M). In contrast, the slow (sustained) I-Ca component was observed at all HPs (-40 to -100 mV) an d had a more positive threshold potential (about -40 mV). This compone nt was insensitive to low concentration of Ni2+ but was sensitive to n ifedipine and BAY K 8644. These results suggest that the fast-inactiva ting (transient) current component might be a T-type Ca2+ current (I-C aT), whereas the sustained component is an L-type Ca2+ current (I-CaL) . The current density of total I-Ca (L + T), I-CaL, and I-CaT declined with aging, but the decline in I-CaT was less.