Evidence that angiotensin II enhances noradrenaline release from sympathetic nerves in mouse atria by activating protein kinase C.

1. Mouse atria were incubated with [3H]-noradrenaline and the outflow of radioactivity induced by electrical field stimulation (5 Hz, 60 s) was used as an index of noradrenaline release. Angiotensin II (1 x 10(-8) M) significantly enhanced the stimulation-induced (S-I) outflow of radioactivity. 2. Phorbol 12-myristate 13-acetate (0.001-1.0 x 10(-6) M) and phorbol 12, 13-dibutyrate (0.001-1.0 x 10(-6) M), protein kinase C activating phorbol esters, significantly enhanced the S-I outflow of radioactivity. Phorbol dibutyrate produced a greater maximal enhancement of S-I outflow of radioactivity than phorbol myristate acetate. The enhancement of S-I outflow of radioactivity produced by the combination of phorbol dibutyrate (1.0 x 10(-7) M) and phorbol myristate acetate (1.0 x 10(-7) M) was no greater than that produced by phorbol dibutyrate (1.0 x 10(-7) M) alone. The enhancement of S-I outflow of radioactivity produced by phorbol myristate acetate (1.0 x 10(-7) M) was constant whether the tissue was exposed for 15, 45 or 75 min. 3. When angiotensin II (1.0 x 10(-8) M) was present with the maximally effective concentration of phorbol dibutyrate (1.0 x 10(-7) M) it did not increase S-I outflow of radioactivity. 8-bromo-cyclic AMP (9.0 x 10(-5) M) by itself increased the S-I outflow of radioactivity and in the presence of the maximally effective concentration of phorbol dibutyrate the enhancement of S-I outflow of radioactivity produced by 8-bromo-cyclic AMP was maintained. 4. A protein kinase inhibitor, K-252a (1.0 x 10(-6) M), did not affect S-I outflow of radioactivity. K-252a significantly reduced the enhancement of S-I outflow of radioactivity produced by both phorbol myristate acetate (0.03 or 0.1 x 10(-6) M) and phorbol dibutyrate (0.01 or 1.0 x 10(-6) M). 5. K-252a (1.0 x 10(-6) M) blocked the enhancement of S-I outflow of radioactivity produced by angiotensin II (1.0 x 10(-8) M) and tetraethylammonium (1.0 x 10(-4) M). 6. These results suggest that angiotensin II receptors may enhance noradrenaline release through the pool of protein kinase C that is activated by phorbol dibutyrate.