Effects of cyclic AMP and protein kinase on calcium uptake in a microsomal fraction from guinea pig taenia caecum.

A microsomal fraction was isolated from guinea pig taenia caecum by differential centrifugation. Activities of ouabain-sensitive (Na+, K+)-ATPase, 5'-nucleotidase and NADPH-cytochrome c reductase were enriched in the microsomal fraction. On the other hand, less cytochrome c oxidase and monoamine oxidase were contained in this fraction. These results suggest that the microsomal fraction used in this study was derived from both sarcolemma and sarcoplasmic reticulum. Ca2+ uptake by this fraction was strictly dependent on the presence of ATP and was facilitated by oxalate. An ATP-regenerating system was required for the determination of Ca2+ uptake, when a lower concentration of ATP (e.g. 0.25 mM) was used. Phosphorylation of the microsomal fraction was doubled when these membranes were incubated in the presence of cyclic AMP plus cyclic AMP-dependent protein kinase (protein kinase). When the microsomal fraction was pretreated with cyclic AMP plus protein kinase, Ca2+ uptake was stimulated. The increases in microsomal phosphorylation and Ca2+ uptake were significantly correlated (P less than 0.01). This stimulation of Ca2+ uptake by microsomal phosphorylation was observed only in the presence of protein kinase, oxalate, and low Ca2+ and Mg2+ concentrations. The results suggest that stimulation of Ca2+ uptake may be the mechanism by which cyclic AMP is involved in beta-adrenergic relaxation of smooth muscle.