Submitted on August 30, 2005
Accepted on November 10, 2005
ROLE OF PLASMA MEMBRANE Ca2+-ATPase IN CONTRACTION-RELAXATION PROCESSES OF THE BLADDER: EVIDENCE FROM PMCA GENE-ABLATED MICE
Li Liu1, Yukisato Ishida1, Gbolahan Okunade2, Gary E Shull2, and Richard J Paul1*
1 Molecular & Cellular Physiology, University of Cincinnati, Cincinnati, OH, USA
2 Molecular Genetics, Biochemistry and Microbiology, University of Cincinnati, Cincinnati, OH, USA
* To whom correspondence should be addressed. E-mail: Richard.Paul{at}uc.edu.
We investigated the role and relation of the plasma membrane Ca2+ ATPase (PMCA), sarco-(endo)plasmic reticulum ATPase (SERCA2) and Na+/Ca2+ exchanger (NCX) in bladder smooth muscle contractility using Pmca gene-ablated mice: Pmca4 null mutant mice (Pmca4-/-) and heterozygous Pmca1 and homozygous Pmca4 double gene-targeted mice (Pmca1+/-Pmca4-/-). The gene manipulation did not alter the amounts of PMCA1, SERCA2 and NCX. To study the role of each Ca2+-transport system, contraction of circular ring preparations were elicited with KCl (80 mM) plus atropine, then the muscle was relaxed with Ca2+-free PSS containing EGTA. We measured the contributions of Ca2+ clearance components by inhibiting SERCA2 with cyclopiazonic acid (10 µM) and/or NCX by replacing NaCl with methyl-glucamine/HCl plus KB-R7943 (10 µM). Contraction half-time (time to 50% of maximum tension) was prolonged in the gene-targeted muscles, but marginally shortened when inhibited SERCA2 or NCX. 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 non-stimulated 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 25~30%, SERCA2 20% and NCX 70%. PMCA and SERCA2 appeared to function additively, but the function of NCX might overlap with those of other components. In summary, the gene manipulation of PMCA indicates that PMCA, in addition to SERCA2 and NCX plays a significant role in both excitation-contraction coupling and the Ca2+-extrusion-relaxation relationship, i.e. Ca2+ homeostasis, of bladder smooth muscle.
