Two-step Ca2+ intracellular release underlies excitation-contraction coupling in mouse urinary bladder myocytes

Abstract

The relative contributions of Ca2+-induced Ca2+ release (CICR) versus Ca2+ influx through voltage-dependent Ca2+ channels (VDCCs) to excitation-contraction coupling has not been defined in most smooth muscle cells (SMCs). The present study was undertaken to address this issue in mouse urinary bladder (UB) smooth muscle cells (UBSMCs). Confocal Ca2+ images were obtained under voltage- or current-clamp conditions. When UBSMCs were activated by a 30-ms depolarization to 0 mV, intracellular Ca2+ concentration ([Ca2+]i) increased in several small, discrete areas just beneath the cell membrane. These Ca2+ “hot spots” then spread slowly through the myoplasm as Ca2+ waves, which continued even after repolarization. Shorter depolarizations (5 ms) elicited only a few Ca2+ sparks, which declined quickly. The number of Ca2+ sparks, or hot spots, was closely related to the depolarization duration in the range of ∼5–20 ms. There was an apparent threshold depolarization duration of ∼10 ms within which to induce enough Ca2+ transients to spread globally and then induce a contraction. Application of 100 μM ryanodine to the pipette solution did not change the resting [Ca2+]i or the VDCC current, but it did abolish Ca2+ hot spots elicited by depolarization. Application of 3 μM xestospongin C reduced ACh-induced Ca2+ release but did not affect depolarization-induced Ca2+ events. The addition of 100 μM ryanodine to tissue segments markedly reduced the amplitude of contractions triggered by direct electrical stimulation. In conclusion, global [Ca2+]i rise triggered by a single action potential is not due mainly to Ca2+ influx through VDCCs but is attributable to the subsequent two-step CICR.