Protein kinase C regulates the tonic but not the phasic component of contraction in guinea-pig ileum

We have investigated the effect of phorbol esters and the down-regulation of protein kinase C on contraction of guinea-pig ileum longitudinal smooth muscle to carbachol and high K⁺. Phorbol 12,13-dibutyrate (PDBu) enhanced the phasic component and inhibited or enhanced, respectively, the tonic component of contraction to carbachol and high K⁺. In contrast, 4α-phorbol, which does not activate protein kinase C, had no effect on these responses. Exposure to phorbol 12-myristate 13-acetate (PMA; 1 μM) for up to 8 h induced a time-dependent loss of [³H]-PDBu binding sites, consistent with the down-regulation of protein kinase C by this treatment. The phasic component of contraction to carbachol or high K⁺ was unaffected following the down-regulation of protein kinase C. The tonic component of contraction to carbachol was markedly enhanced by this treatment while that to high K⁺ was partially suppressed. These data suggest that although the activation of protein kinase C can lead to potentiation of the phasic component of contraction to carbachol or high K⁺, this appears to have little physiological significance since the response is not altered in tissues in which protein kinase C has been down-regulated. On the other hand, protein kinase C may limit the tonic contraction to carbachol but potentiate that to high K⁺.