Epinephrine as a tool to investigate the question of recycling of sympathetic storage vesicles in the heart: chemical and morphological studies.

The aim of the present investigation was to examine the question of reutilization of norepinephrine storage vesicles in the cardiac sympathetic nerves. More specifically, the question was whether exocytotically depleted stores of norepinephrine could be restored by epinephrine and the restoration of epinephrine could be demonstrated by chemical and morphological examinations. Stimulation of guinea-pig cardiac sympathetic nerves (1 Hz, 15 sec/min for 75 min) in the presence of 20 mM tetraethylammonium lowered norepinephrine fro 2.24 to 0.46 micrograms/g. Exposure of such hearts to 6 or 60 microM epinephrine increased epinephrine contents to 1.4 and 2.2 micrograms/g, respectively. Accumulation of epinephrine by normal or norepinephrine-depleted heart was depressed over 90% by desipramine. If intermittent stimulation was given in the presence of tetraethylammonium for only 10 min, norepinephrine and epinephrine values of epinephrine-loaded hearts were reduced over 50%. Perfusion of such hearts with epinephrine-Krebs' solution raised epinephrine content to 3.3 micrograms/g. Stimulation-evoked overflows of epinephrine after first and second loading with epinephrine were almost identical. Ca-free plus 1 mM ethylene glycol bis)beta-aminoethyl ether)-N,N'-tetracetic acid completely blocked stimulation-evoked overflow of epinephrine. Dense-cored vesicles were observed in sympathetic nerves located in the region of the S-A node. Stimulation of nerves in the presence of tetraethylammonium resulted in almost complete disappearance of dense-cored vesicles. Perfusion of such treated hearts with epinephrine led to reappearance of dense-cored vesicles. The number of dense-cored vesicles after exposure to epinephrine was, in fact, greater than that seen in the control tissue. Taken together, our data provide additional support to the concept that storage vesicles are reused in sympathetic nerves of the guinea-pig heart.