Phorbol esters elicit Ca²⁺-dependent delayed contractions in diabetic rat aorta

Abstract

To determine whether diabetes alters vascular effects mediated by activation of protein kinase C, the contractions induced by phorbol esters were examined in aortic rings from rats with 8- to 12-week streptozotocin-induced diabetes and compared with those from age-matched control rats. In diabetic rat aorta, phorbol 12,13-dibutyrate (PDB) (≥ 30 nM) and 12-O-tetrade-canoylphorbol 13-acetate (TPA) (300 nM) elicited a delayed, sharply developing rise in tension following an initial gradually developing contraction. In control rat aorta, these agents produced only an initial slowly developing contraction. Both the initial and the delayed contractile responses observed in diabetic aorta were completely abolished by pretreatment with 20 nM staurosporine, and the delayed phase of contraction was not seen in Ca²⁺-free medium or in the presence of 1 μM nifedipine. The concentration-response curves for the contractions induced by PDB revealed that PDB at concentrations ≥ 30 nM produced significantly greater responses in diabetic aorta than in control aorta. In control aorta, exposure to Ca²⁺-free medium and pretreatment with 1 μM nifedipine shifted the concentration-response curves for PDB to the right without changing the maximal response. Under these conditions, there were no differences in the curves for PDB in control and diabetic aortas. These results suggest that the appearance of the delayed phase of contraction, possibly due to a delayed opening of Ca²⁺ channels, during activation of protein kinase C may be responsible for the enhanced contractile responses to phorbol esters in diabetic rat aorta.