Phorbol 12,13-dibutyrate, an activator of protein kinase C, stimulates both contraction and Ca²⁺ fluxes in dog saphenous vein

Summary

The vascular effects of phorbol 12,13-dibutyrate (PDBu) were studied in the dog saphenous vein. PDBu (1 μM) caused contraction (0.58 ± 0.22 g/mg wet wt.) and Ca uptake (74.2 ± 41.2 μmol/kg wet wt.) which were unaffected by 10 μM phentolamine (N = 6). The PDBu-induced contraction was greatly (60–80%) inhibited in Ca²⁺-free solution. 15 Ca efflux measurements performed in Ca²⁺-free solution showed that PDBu did not cause Ca release from intracellular storage sites. The contractile response to PDBu (1 nM-1 μM) was significantly correlated with the magnitude of Ca uptake; contraction and the rise in tissue Ca²⁺ also had a similar time course. Correlation between the two measures persisted when the responses to PDBu were augmented by co-administration with 20 mM KCl. However, no synergism occurred between the two agonists. Both the contraction and Ca uptake responses to PDBu were reduced by nifedipine and verapamil, each at 1 μM. In the Triton X-100 skinned saphenous vein, where the voltage-dependent Ca channel is not functional, 10 μM PDBu did not cause contractions in the presence of 0.1 μM Ca²⁺. Thus, contraction of the intact saphenous vein by PDBu characteristically exhibits great Ca dependence and PDBu seems to activate the voltage-dependent Ca channel, presumably through stimulation of protein kinase C; the ensuing Ca entry is primarily responsible for contraction. However, the mechanism responsible for the PDBu-induced contractions that are resistant to Ca²⁺-free PSS or Ca entry blockers remains to be defined. It appears that the dog saphenous vein differs from dog femoral artery, rabbit aorta and pig carotid artery where PDBu contractions do not display dependence on external Ca²⁺.