Neurogenic and Humoral Factors Controlling Vascular Resistance in the Spontaneously Hypertensive Rat

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Abstract
The mechanism of sustained hypertension in spontaneously hypertensive rats has not been elucidated. In the present investigation, vasoconstrictor responses to a variety of neurogenic and humoral interventions were studied in the perfused hindquarters of Okamoto spontaneously hypertensive rats and normotensive Wistar rats. In addition, central sympathetic electrical discharge was measured. Vasoconstrictor responses in the hindquarters to lumbar sympathetic nerve stimulation were unchanged or reduced in the spontaneously hypertensive rats, but the responses to intra-arterially administered norepinephrine and epinephrine were enhanced. Vascular responses to intra-arterially administered tyramine, angiotensin, and barium chloride were also greater in the spontaneously hypertensive rats. Vascular resistance was significantly higher in the spontaneously hypertensive rats, and this difference remained following bilateral lumbar sympathectomy. Despite elevated systemic blood pressure, integrated nerve activity at rest was not different in the spontaneously hypertensive rats. The inverse relationship between arterial blood pressure and sympathetic nerve discharge was not different between spontaneously hypertensive and control rats when pressure was either raised or lowered. Changes in efferent sympathetic discharge produced by activation of chemoreceptors (asphyxia) were somewhat less in the spontaneously hypertensive rats. The contribution of low-pressure baroreceptors (45° tilt) to activation of sympathetic vasomotor tone was not different in the spontaneously hypertensive rats despite a greater decline in systemic blood pressure during the procedure. These data demonstrate that established hypertension in the spontaneously hypertensive rat does not derive from either enhanced central adrenergic discharge or altered central integration of afferent information from peripheral sensory receptors but may result from humoral (e.g., increased reactivity to vasoconstrictors) or structural factors.