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Hypotensive effect of push-pull gravitational stress occurs after autonomic blockade

Department of Exercise Science, The University of Iowa, Iowa City, Iowa 52242

Submitted 19 February 2003 ; accepted in final form 12 March 2003

The "push-pull" effect denotes the reduced tolerance to +G_z (hypergravity) when +G_z stress is preceded by exposure to hypogravity, i.e., fractional, zero, or negative G_z. Previous studies have implicated autonomic reflexes as a mechanism contributing to the push-pull effect. The purpose of this study was to test the hypothesis that nonautonomic mechanisms can cause a push-pull effect, by using eye-level blood pressure as a measure of G tolerance. The approach was to impose control (30 s of 30° head-up tilt) and push-pull (30 s of 30° head-up tilt immediately preceded by 10 s of -15° headdown tilt) gravitational stress after administration of hexamethonium (10 mg/kg) to inhibit autonomic ganglionic neurotransmission in four dogs. The animals were chronically instrumented with arterial and venous catheters, an ascending aortic blood flow transducer, ventricular pacing electrodes, and atrioventicular block. The animals were paced at 75 beats/min throughout the experiment. The animals were sedated with acepromazine and lightly restrained in lateral recumbency on a tilt table. After the onset of head-up tilt, the magnitude of the fall in eye-level blood pressure from baseline was -27.6 ± 2.3 and -37.9 ± 2.7 mmHg for the control and push-pull trials, respectively (P < 0.05). Cardiac output fell similarly in both conditions. Thus a push-pull effect attributable to a rise in total vascular conductance occurs when autonomic function is inhibited.

arterial blood pressure; tilt; orthostatic stress; atrioventricular block; dog

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