CALCIUM CURRENTS IN HUMAN AND CANINE JEJUNAL CIRCULAR SMOOTH-MUSCLE CELLS

Background & Aims: Although calcium plays an essential role in intestinal smooth muscle contractile activity, calcium entry pathways in canine and human small intestine are largely unknown. The goal of this study was to characterize calcium channels, a potential entry pathway for calcium, in isolated circular smooth muscle cells of canine and human jejunum. Methods: Single freshly dissociated human and canine jejunal circular smooth muscle cells were studied using single-channel and perforated whole-cell patch clamp recordings as well as fluorescence dual wavelength ratio imaging. Results: An inward whole-cell current was identified that was carried by a 17 pS (80 mmol/l Ba2+) dihydropyridine-sensitive, barium-permeable channel. The current was potentiated by BayK 8644 (1 mu mol/L; n = 3; 82% +/- 34%), acetylcholine (1 mu mol/L; n = 8; 42% +/- 5%), and erythromycin (1 mu mol/L; n = 9; 70% +/- 11%) and was completely blocked by nifedipine (1 mu mol/L; n = 6) or diltiazem (200 mu mol/L; n = 4). Application of BayK 8644 (1 mu mol/L), acetylcholine (1 mu mol/L), or erythromycin (1 mu mol/L) to Fura-2-loaded smooth muscle cells bathed in Krebs' solution containing 2.54 mmol/L calcium increased intracellular calcium levels. Conclusions: A calcium entry pathway was identified in canine and human jejunal circular smooth muscle cells. The pathway was mediated by a dihydropyridine-sensitive calcium channel. The channel allowed the entry of significant amounts of calcium at physiological extracellular calcium concentrations.