Depletion of either ryanodine- or IP₃-sensitive calcium stores activates capacitative calcium entry in mouse anococcygeus smooth muscle cells

Abstract

 The properties of the calcium stores coupled to a depletion-operated cation current (I _DOC) proposed to underlie capacitative calcium entry were studied in single smooth muscle cells isolated from the mouse anococcygeus using the whole-cell patch-clamp technique. Both caffeine (10 mM) and carbachol (50 μM) activated an initial, large (≈ 200 pA), transient, calcium-dependent chloride current (I _ClCa) followed by a smaller (≈ 10 pA) sustained, non-selective cation current (I _DOC). Intracellular application of heparin (5 mg ml^–1) abolished the response to carbachol but potentiated that to caffeine. Ryanodine (3 μM) activated I _DOC but not I _ClCa; ryanodine (30 μM) failed to produce any response. Both concentrations of ryanodine abolished the response to caffeine and prevented activation of I _ClCa by carbachol. In the presence of 30 μM, but not 3 μM, ryanodine, carbachol was able to activate I _DOC. Cyclopiazonic acid (CPA; 10 μM) abolished the response to carbachol; however, caffeine was still able to activate I _ClCa. In whole-muscle tension recordings, ryanodine at both 3 and 30 μM produced contractions of the tissue but only that in response to the lower concentration was maintained. Thus, depletion of either inositol 1,4,5-trisphosphate-(IP₃-) sensitive or ryanodine-sensitive calcium stores is able to activate I _DOC, and, by extension, capacitative calcium entry in this tissue.