Role of plasma membrane Ca2+-ATPase in contraction-relaxation processes of the bladder: evidence from PMCA gene-ablated mice

Abstract

We investigated the roles and relationships of plasma membrane Ca(2+)-ATPase (PMCA), sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA)2, and Na(+)/Ca(2+) exchanger (NCX) in bladder smooth muscle contractility in Pmca-ablated mice: Pmca4-null mutant (Pmca4(-/-)) and heterozygous Pmca1 and homozygous Pmca4 double gene-targeted (Pmca1(+/-)Pmca4(-/-)) mice. Gene manipulation did not alter the amounts of PMCA1, SERCA2, and NCX. To study the role of each Ca(2+) transport system, contraction of circular ring preparations was elicited with KCl (80 mM) plus atropine, and then the muscle was relaxed with Ca(2+)-free physiological salt solution containing EGTA. We measured the contributions of Ca(2+) clearance components by inhibiting SERCA2 (with 10 microM cyclopiazonic acid) and/or NCX (by replacing NaCl with N-methyl-D-glucamine/HCl plus 10 microM KB-R7943). Contraction half-time (time to 50% of maximum tension) was prolonged in the gene-targeted muscles but marginally shortened when SERCA2 or NCX was inhibited. The inhibition of NCX significantly inhibited this prolongation, suggesting that NCX activity might be augmented to compensate for PMCA4 function in the gene-targeted muscles under nonstimulated conditions. Inhibition of SERCA2 and NCX as well as gene targeting all prolonged the relaxation half-time. The contribution of PMCA to relaxation was calculated to be approximately 25-30%, with that of SERCA2 being 20% and that of NCX being 70%. PMCA and SERCA2 appeared to function additively, but the function of NCX might overlap with those of other components. In summary, gene manipulation of PMCA indicates that PMCA, in addition to SERCA2 and NCX, plays a significant role in both excitation-contraction coupling and the Ca(2+) extrusion-relaxation relationship, i.e., Ca(2+) homeostasis, of bladder smooth muscle.