Evidence from PMCA gene‐ablated mice for distinct roles of PMCA 1 & 4 isoforms: PMCA4 linked with ACh‐receptor function in Ca 2+ ‐homeostasis of bladder smooth muscle

Abstract

We studied Plasma Membrane Ca2+-ATPase (PMCA) isoforms 1 and 4 in bladder smooth muscle Ca2+ homeostasis and contractility using Pmca1+/−, Pmca4−/− and Pmca1+/−Pmca4−/− double gene-targeted mice. The contribution of PMCA to relaxation was calculated to be 25~30%. When [Ca2+]i and contractility were simultaneously measured, KCl elicited both larger forces and Ca2+ in Pmca1+/− and Pmca1+/−Pmca4−/−. The responses to carbachol (CCh) were also greater in Pmca1+/−. In contrast, the responses in Pmca4−/− and Pmca1+/−Pmca4−/− to CCh were significantly smaller. Both the rise and fall half-times of force and [Ca2+]i were prolonged in Pmca4−/− and Pmca1+/−Pmca4−/−, but not in Pmca1+/−. Our evidence indicates distinct isoform functions with PMCA1 involved in overall Ca2+-clearance, while PMCA4 is essential for the [Ca2+]i and contractile responses to CCh. We hypothesized that a localized increase in [Ca2+]i may activate BK+Ca channels or inhibit L-type Ca2+ channels or CCE, leading to lower CCh force. Our data show that neither BK+Ca channels nor CCE were altered, but nifedipine inhibition of CCh force was significantly reduced in Pmca4−/−. Western blot analysis indicated the expression of L-type Ca2+ channels was not changed. These data suggest that the activities of ACh-receptor mediated signal transduction and L- type Ca2+ channel activity may be disrupted by localized increases in [Ca2+]i due to loss of PMCA4. Supported by NIH 61974 (GES, RJP).