Phorbol esters inhibit smooth muscle contractions through activation of Na⁺-K⁺-ATPase

1. The role of protein kinase C (PKC) in agonist-induced contractions of guinea-pig ileum longitudinal smooth muscle has been investigated. 2. The phorbol esters, phorbol 12,13-dibutyrate (PDBu), phorbol 12,13-diacetate (PDA) and phorbol 12-myristate 13-acetate (PMA), relaxed tissues precontracted by submaximal concentrations of carbachol, histamine or substance P. 3. This inhibitory action of the phorbol esters was reversed following the application of ouabain, a specific inhibitor of Na⁺-K⁺-ATPase. Similarly, pretreatment with ouabain inhibited the ability of phorbol esters to relax tissues precontracted by the above agonists. 4. The slow relaxation of the tonic component of contraction induced by submaximal concentrations of carbachol and histamine, and all concentrations of substance P, was abolished in the presence of ouabain. 5. In Na⁺-loaded tissues, PDBu and carbachol caused a concentration-dependent increase of Na⁺-K⁺-ATPase activity, assessed by ouabain-sensitive ⁸⁶Rb⁺-uptake. Extrusion of Na⁺, assessed by the cellular content of the ion, was also stimulated by PDBu (the effect of carbachol was not investigated). 6. We conclude that phorbol esters inhibit the tonic component of contractions induced by submaximal concentrations of these agonists through activation of Na⁺-K⁺-ATPase. We suggest that PKC may exert feedback control over the tonic component of agonist contractions through stimulation of the pump.