Ryanodine receptor type 2 deficiency changes excitation-contraction coupling and membrane potential in urinary bladder smooth muscle

The possibility that the ryanodine receptor type 2 (RyR2) can function as the major Ca2+-induced Ca2+ release (CICR) channel in excitation-contraction (E-C) coupling was examined in smooth muscle cells (SMCs) isolated from urinary bladder (UB) of RyR2 heterozygous KO mice (RyR2+(/-)). RyR2 mRNA expression in UB from RyR2+(/-) was much lower than that in wild-type (RyR2(plus;/plus;)). In single UBSMCs from RyR2(plus;/+), membrane depolarization under voltage clamp initially induced several local Ca2+ transients (hot spots) in peripheral areas of the cell. Then, Ca2+ waves spread from Ca2+ hot spots to other areas of the myocyte. The number of Ca2+ hot spots elicited by a short depolarization (< 20 ms) in UBSMCs of RyR2+(/-) was significantly smaller than in those of RyR2(+/+). The force development induced either by direct electrical stimulation or by 10 mu M acetylcholine in tissue segments of RyR2+(/-) was smaller than and comparable to those in RyR2(+/+), respectively. The frequency of spontaneous transient outward currents in single myocytes and the membrane depolarization by 1 mu M paxilline in tissue segments from RyR2(+/-) were significantly lower and smaller than those in RyR2(+/+), respectively. The urination frequency and volume per voiding in RyR2(+/-) were significantly increased and reduced, respectively, compared with RyR2(+/+). In conclusion, RyR2 plays a crucial role in the regulation of CICR during E-C coupling and also in the regulation of resting membrane potential, presumably via the modulation of Ca2+-dependent K+ channel activity in UBSMCs and, thereby, has a pivotal role in the control of bladder activity.