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

The Na⁺-K⁺-ATPase (NKA) can affect intracellular Ca²⁺ concentration regulation via coupling to the Na ⁺-Ca²⁺ exchanger and may be important in myogenic tone. We previously reported that in mice carrying a transgene for the NKA α₂-isoform in smooth muscle (α_2sm+), the α₂-isoform protein as well as the α₁-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 Ca²⁺ clearance. To elucidate the mechanisms underlying this faster relaxation, we therefore measured the expression and distribution of proteins involved in Ca²⁺ clearance. Na⁺-Ca²⁺ exchanger, sarco(endo)plasmic reticulum Ca²⁺-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 Ca²⁺ clearance mRNA levels were unchanged. Immunocytochemical data indicate that the Ca²⁺ 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 Ca²⁺ 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 Ca²⁺ clearance system.