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Release of Histamine by Sympathetic Nerve Stimulation in the Guinea Pig Heart and Modulation of Adrenergic Responses: A Physiological Role for Cardiac Histamine?

Histamine has been reported to attenuate adrenergic responses in cardiovascular tissues. In guinea pig atria preloaded with [3H]norepinephrine, histamine diminishes the field stimulation-induced efflux of radioactivity; this effect has been attributed to an inhibition of norepinephrine release from nerve endings. To assess the possible physiological relevance of these findings, we have reinvestigated the effects of histamine on cardiac sympathetic responses and on the release of endogenous norepinephrine in the guinea pig heart isolated with its intact sympathetic innervation. Heart rate, left ventricular contractile force, and perfusion pressure all increased with increasing frequencies of sympathetic nerve stimulation (2–8 Hz). Histamine (3 × 10−8 to 3 × 10−7 M) caused dose-dependent attenuation of the responses to sympathetic stimulation. The ability of histamine to modulate nerve stimulation-induced norepinephrine overflow into the coronary effluent was dependent on whether the heart had been preloaded with norepinephrine. Whereas histamine did not cause a significant reduction in nerve stimulation-induced norepinephrine overflow in hearts from untreated animals, histamine significantly reduced stimulation-induced norepinephrine overflow in hearts from guinea pigs that had been pretreated with norepinephrine before sacrifice. Histamine also attenuated the increases in left ventricular contractile force, perfusion pressure, and heart rate, which result from the intracardiac administration of norepinephrine (0.16-^g bolus injection). In this respect, histamine was as effective as it was in inhibiting the responses elicited by nerve stimulation. Thus, in normal animals, the negative modulatory effect of histamine on adrenergic responses can be attributed largely, if not totally, to a postjunctional mechanism. In contrast, a prejunctional action of histamine may contribute significantly to the negative modulation observed in norepinephrine-preloaded hearts. Since we have observed a large increase in the amount of endogenous histamine present in the coronary effluent after sympathetic stimulation (930 pg during the 30 seconds poststimulation vs. 240 pg during 30 seconds prestimulation), as well as a prolongation of nerve stimulation-induced cardiac responses in the presence of the H2 receptor antagonist tiotidine, we postulate that histamine plays a physiological role as a modulator of sympathetic responses in the heart.