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Fine structural identification of a reserpine-resistant norepinephrine store in adrenergic nerve terminals in guinea-pig atria.

Administration of small doses (10 or 100 µg/kg/day) of reserprine for 7 days to adult guinea pigs resulted in a marked depletion of right atrial norepinephrine content without proportional changes in cardiac responses to accelerans nerve stimulation or to tyramine. Catecholamine fluorescence histochemistry and electron microscopy were used in order to identify morphologically this reserpine-resistant norepinephrine store. Virtually no adrenergic nerve terminals were found on fluorescence histochemical examination of atria from reserpine-treated animals which indicates that the amounts of adrenergic transmitter were below the detection limit of this method. Upon electron microscopic examination of atria fixed in potassium permanganate, a small but statistically significant number of adrenergic nerves, identifiable by the presence of small granular synaptic vesicles, were observed in the atria of guinea pigs treated with reserpine. This study demonstrates that the reserpine-resistant norepinephrine store need not be qualitatively different from the principal norepinephrine store in adrenergic nerve terminals.