Cardiopulmonary and carotid baroreflex control of splanchnic and forearm circulations.

The objective was to determine whether a rise in carotid sinus transmural pressure by neck suction (NS) would counteract vasoconstriction secondary to inhibition of discharge of arterial and cardiopulmonary baroreceptors by simultaneous lower body negative pressure (LBNP). NS alone was applied to seven normal human subjects at -40 mmHg for 400-600 ms at each heartbeat during a 6-min period. NS reduced mean arterial pressure (MAP) from 94 +/- 6 to 86 +/- 9 mmHg and heart rate (HR) from 64 +/- 5 to 60 +/- 4.7 beats/min but did not affect vascular resistance in the splanchnic region (flow by constant infusion of indocyanine green; assumed constant extraction) or in the forearm (venous occlusion plethysmography). The same NS stimulus was applied during 23 min of continuous LBNP at -40 mmHg. LBNP alone before NS significantly reduced central venous pressure (CVP) from 5 +/- 0.3 to 1 +/- 0.5 mmHg and raised splanchnic (+34%) and forearm (+70%) vascular resistances and HR (from 64 to 74 beats/min) without reducing MAP. NS plus LBNP reduced MAP from 103 +/- 8 to 95 +/- 6 mmHg and HR from 74 +/- 6 to 67 +/- 5 beats/min without changing CVP but did not alter vascular resistances, which remained elevated and constant throughout LBNP before and after NS. Increments in plasma concentrations of renin (240%), aldosterone (70%), epinephrine (112%), and norepinephrine (46%) accompanied LBNP and NS; a separate influence of NS was not discernible. We conclude that vasoconstriction in response to combined cardiopulmonary and aortic inhibition is not overpowered by carotid sinus stimulation.