NE Release from Cardiac Synaptosomes

Sodium nitroprusside (SNP), hydrocortisone 21-hemisuccinate, desipra-mine hydrochloride, L-arginine hydrochloride, and w-conotoxin GVIA were purchased from Sigma Chemical Company (St. Louis, MO). Acid-washed alumina, sodium octyl sulfate, and catecholamine standards were purchased 

Griffith (Medical College of Wisconsin Milwaukee, WI). Compound Ro31-8220 was a gift of Dr, T. J. Hallam (Roche Research Centre, Welwyn Gardens City, Herefordshire, England). 

All data were expressed as means SEM. The Student s t test was used when two groups of data were compared. For multiple comparisons the Newman-Keuls test was used. Other statistical tests, when used, are indicated in the appropriate figure legends. A value of P 0.05 was considered statistically significant. 

We first assessed whether NO affects sympathetic neurotransmission in a peripheral vascular resistance district, namely, the mesenteric arterial bed of the rat. As shown in Fig. 1 A, electrical stimulation of periarterial adrenergic nerves at 2-8 Hz for 30 sec elicited a frequency-dependent 7-73% increase in perfusion pressure (i.e., vasoconstriction). This was accompanied by a frequency-dependent increase in NE overflow (Fig. 1C), which peaked in the first 30 sec following the termination of electrical stimulation and declined over the next 5-10 min. The NO donor SNP, at 0.1 and 1 //.M, significantly depressed the vasoconstriction elicited by electrical stimulation and by the administration of phenylephrine (Fig. lA). In contrast, SNP did not modify the increase in NE overflow (Fig. 1C). These findings indicate that SNP (i.e., exogenous NO) attenuates the vasoconstricting response to sympathetic nerve stimulation by a postjunctional action. 

FIGURE 2 Effects of sodium nitroprusside (SNP 1 /xM) and N -niethyl-L-arginine (NMA 300 /xM), alone or in the presence of an L-arginine (Arg) excess (1.2 raM), on the vasoconstriction (A) and norepinephrine (NE) release (B) evoked by adrenergic nerve stimulation (30 sec at 4 Hz) in the mesenteric arterial bed of the rat perfused with the o 2-adrenoeeptor antagonist yohimbine (1 /aAl). Values are expressed as the means of six experiments.. Significantly different from control values by the Newman-Keuls test at P 0.05. For other details see the legend to Fig. 1. 

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Interestingly, we found that the same concentrations of SIN-1, which enhanced NE overflow in normal and ischemic hearts, in fact, decreased K -induced NE release from cardiac synaptosomes (cf. Figs. 4 and 6 with Fig. 7). In the altered chemical environment that exists in the synaptosomal preparation as compared to the intact heart, it is conceivable that ONOO may have a differential ability to decompose into NO and/or nitrosate thiols (Stamler et al., 1992), thereby explaining the difference in bioactivity. 

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