Hypotonic swelling increases L-type calcium current in smooth muscle cells of the human stomach

The purpose of the present study was to characterize the Ca²⁺ channels in smooth muscle cells from human stomach and to examine the effects of osmotic swelling on the channel activity. Ca²⁺ channel current with either Ca²⁺ or Ba²⁺ as charge carrier was recorded from freshly isolated smooth muscle cells using the conventional whole-cell patch clamp technique. The degree of cell swelling as a result of hypotonic challenge was monitored using a video image analysis system. The changes in intracellular Ca²⁺ concentration ([Ca²⁺](i)) were measured by microfluorimetry. The pharmacological and voltage activation profile suggests a typical dihydropyridine-sensitive L-type Ca²⁺ current. Cell swelling, induced by hypotonic challenge, enhanced the amplitude of currents through L-type Ca²⁺ channels without significant effects on steady-state voltage dependency. After treatment with the L-type Ca²⁺ channel agonist Bay K 8644 (0.1-2 μm), no further significant increase in calcium channel current or corresponding [Ca²⁺](i) transients were provoked by the swelling. The above results demonstrated that the presence of L-type Ca²⁺ current in smooth muscle cells of the human stomach and the augmentation of the current are closely associated with the volume increase resulting from hypotonic swelling.