Na+-K+-ATPase and Ca2+ clearance proteins in smooth muscle: a functional unit

The Na + -K + -ATPase (NKA) can affect intracellular Ca 2+ concentration regulation via coupling to the Na + -Ca 2+ exchanger and may be important in myogenic tone. We previously reported that in mice carrying a transgene for the NKA α 2 -isoform in smooth muscle (α 2sm+ ), the α 2 -isoform protein as well as the α 1 -isoform (not contained in the transgene) increased to similar degrees (2–7-fold). Aortas from α 2sm+ mice relaxed faster from a KCl-induced contraction, hypothesized to be related to more rapid Ca 2+ clearance. To elucidate the mechanisms underlying this faster relaxation, we therefore measured the expression and distribution of proteins involved in Ca 2+ clearance. Na + -Ca 2+ exchanger, sarco(endo)plasmic reticulum Ca 2+ -ATPase (SERCA), and plasma membrane Ca 2+ -ATPase (PMCA) proteins were all elevated up to approximately fivefold, whereas actin, myosin light chain, and calponin proteins were not changed in smooth muscle from α 2sm+ mice. Interestingly, the corresponding Ca 2+ clearance mRNA levels were unchanged. Immunocytochemical data indicate that the Ca 2+ clearance proteins are distributed similarly in wild-type and α 2sm+ aorta cells. In studies measuring relaxation half-times from a KCl-induced contraction in the presence of pharmacological inhibitors of SERCA and PMCA, we estimated that together these proteins were responsible for ∼60–70% of relaxation in aorta. Moreover, the percent contribution of SERCA and PMCA to relaxation rates in α 2sm+ aorta was not significantly different from that in wild-type aorta. The coordinate expressions of NKA and Ca 2+ clearance proteins without change in the relative contributions of each individual protein to smooth muscle function suggest that NKA may be but one component of a larger functional Ca 2+ clearance system.