Ca²⁺ channel ligand sensitive responses to the phorbol ester 12-O-tetradecanoylphorbol 13-acetate in vascular smooth muscle

The action of a tumor-promoting phorbol ester, 12-O-tetradecanoylphorbol 13-acetate (TPA), on isolated rat aortic and tail artery strips has been characterized. TPA (10^-9 - 10^-7 M) produced a graded contraction developing maximum tension over 30-40 min. The contraction was irreversible and was not relaxed by prolonged washing with physiologic saline. Relaxation occurred upon washing with Ca²⁺-free saline but readdition of Ca²⁺ restored response. TPA was without significant effect in rat tail arteries in physiologic saline but produced responses in saline containing elevated K⁺ (15 mM). The protein kinase C inhibitor, CP-46,665-1 (4-aminomethyl-1-[2,3-(di-n-decyloxy)n-propyl]-4-phenylpiperidine dihydrochloride) (5 x 10^-5 M), blocked the response to TPA but was without effect on responses to Bay K 8644 (2,6-dimethyl-3-carbomethoxy-5-nitro-4-(2-trifluoromethylphenyl) 1,4-dihydropyridine), KCl, phenylephrine, and B-HT 920 (6-allyl-2-amino-5,6,7,8-tetrahydro-4H-thiazolo[4,5-d]azepin dihydrochloride). The calcium channel antagonist nifedipine and its analogue, 2,6-dimethyl-3,5-dicarbomethoxy-4-(3-cyanophenyl)-1,4-dihydropyri dine, inhibited TPA responses with IC₅₀ values of 9.28 x 10^-9 and 1.96 x 10^-7 M, respectively. Responses to Bay K 8644 in rat aorta were maximum in the presence of elevated KCl (10 mM), but TPA at concentrations of 10^-9 and 3 x 10^-9 M potentiated responses to Bay K 8644 in physiologic saline to levels approximating those in elevated K⁺ saline. TPA similarly potentiated responses to Ca²⁺ in Ca²⁺-free solution. In the presence of TPA, 10^-8 and 3 x 10^-8 M, responses to Ca²⁺ in nondepolarizing saline were potentiated to levels seen under depolarizing conditions. The present results suggest a relationship between protein kinase C and Ca²⁺ channel activation. However, alternative possibilities, including enhancement of the Ca²⁺ sensitivity of the contractile apparatus, may also contribute to the observed effects.