M-2 receptor activation of nonselective cation channels in smooth muscle cells: Calcium and G(i)/G(0) requirements

Muscarinic stimulation of fura 2-loaded smooth muscle cells evoked a rapidly inactivating Ca2+-activated Cl- current [I-Cl(Ca)] and a sustained nonselective cation current (I-cat) as well as a transient (Delta Ca-tran) and a sustained (Delta Ca-sus) elevation of cytosolic Ca2+ concentration ([Ca2+](i)). Caffeine and inositol 1,4,5-trisphosphate induced Delta Ca-tran and I-Cl(Ca) but not I-cat or Delta Ca-sus. M-2 receptor antagonism blocked muscarinic activation of I-cat and Delta Ca-sus but not I-Cl(Ca) and Delta Ca-tran. M-3 antagonism blocked activation of I-Cl(Ca) and I-cat and a rise in [Ca2+](i), but application of caffeine with methacholine restored I-cat and Delta Ca-sus. After depletion of intracellular Ca2+ stores, methacholine failed to induce I-cat or a [Ca2+](i) increase and, in pertussis toxin-treated cells, I-Cl(Ca) and Delta Ca-tran but not I-cat or Delta Ca-sus were evoked. Anti-G alpha(i-1)/G alpha(i-2) antibodies and anti-G alpha(i-3)/G(o) alpha antibodies blocked I-cat but did not affect I-Cl(Ca). Anti-G(q) alpha/G alpha(11) antibodies greatly inhibited I-Cl(Ca) but did not affect I-cat. Activation of M-2 receptors leads to the opening of nonselective cation channels through G(i)/G(o) proteins in smooth muscle cells, resulting in a sustained rise in [Ca2+](i). A rise in [Ca2+](i) is necessary but not sufficient for activation of nonselective cation channels.