Effects of propranolol and yohimbine on periarterial nerve stimulation-induced release of endogenous norepinephrine from the mesenteric vasculature of Wistar Kyoto and spontaneously hypertensive rats.

The overflow of endogenous norepinephrine (NE) from the mesenteric vasculature of the isolated mesentery of Wistar Kyoto rats (WKY) and spontaneously hypertensive rats (SHR) was determined in response to periarterial nerve stimulation (PNS) before and after pretreatment with propranolol or yohimbine. Propranolol pretreatment did not significantly alter spontaneous NE overflow, total NE overflow, NE overflow/stimulus or fractional NE overflow in either WKY or SHR mesenteric vascular preparations at any of the PNS frequencies used. Yohimbine pretreatment did not significantly alter spontaneous NE overflow but did significantly increase total NE overflow, NE overflow/stimulus and fractional NE overflow at all PNS frequencies used in both WKY and SHR preparations. The magnitude of the effect of yohimbine on NE overflow/stimulus did not differ between WKY and SHR over the range of PNS frequencies used. The lack of effect of propranolol on NE overflow suggests that corelease of epinephrine is not sufficient to activate beta adrenergic receptor-mediated modulation of noradrenergic neurotransmission in either WKY or SHR mesenteric vascular preparations under the in vitro study conditions used. However, the effects of yohimbine indicate that prejunctional alpha-2 adrenergic receptor-mediated inhibition of noradrenergic neurotransmission is operative in both WKY and SHR mesenteric vascular preparations. These effects of yohimbine also suggest that no significant differences exist between the functional level of prejunctional alpha-2 adrenergic receptor-mediated autoinhibitory modulation of noradrenergic neurotransmission in mesenteric vascular preparations from adult WKY and SHR under the in vitro study conditions used.