Diminished sympathetic silent period in spontaneously hypertensive rats

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Abstract
To determine if elevated sympathetic activity occurs in spontaneous hypertension, the silent period induced in splanchnic nerves following electrical stimulation of dorsal medullary sympathoexcitatory sites was compared in anesthetized normotensive Wistar Kyoto rats (WKY) and Okamoto spontaneously hypertensive rats (SHR). The strength of silent periods was defined as the degree of inhibition of responses to testing stimuli delivered at various latencies following conditioning trains, and it was assumed to be inversely related to the level of sympathetic activity. Weanling SHR exhibited weaker silent periods than weanling WKY although, at that age, the arterial pressures of the strains were not significantly different. Silent periods were also weaker in adult SHR than in adult WKY. This difference persisted after arterial pressures, which fell under anesthesia, were raised by phenylephrine infusions to the respective normal levels in each strain. Elevated sympathetic activity exists during both the development and maintenance of spontaneous hypertension in rats.