The Participation of the Venomotor System in Pressor Reflexes

Abstract

In dogs anesthetized with chloralose, innervated loops of intestine were isolated so as to permit periodic interruptions of blood flow to record pressure-volume diagrams from the venous bed. Pressor reflexes were found to produce alterations in the venous distensibility diagrams which, on the basis of comparison with distensibility changes produced by constrictor drugs, are interpreted as direct evidence of venoconstriction accompanying pressor responses of the cardiovascular system. I NVESTIGATIONS of the functional role of the venomotor system have been handicapped by the lack of an adequate method for assessing its activity. Changes in the caliber of veins have been observed under various conditions, 1 but this is not adequate evidence of venomotor function unless these observations are correlated with an analysis of the associated hemodynamic factors. In the over-all regulation of the cardiovascular system, however, indirect evidence justifies the inference that there must be an important venomotor contribution to circulatory homeostasis, in addition to the well demonstrated adjustments of cardiac output and arteriole resistance. • 3Opportunities for a direct attack on this problem have been developed from studies of the distensibility of the splanchnic venous system 4 and the analysis of the influence of constrictor drugs on this distensibility.5 Such distensibility determinations appear more relevant to the problem of venomotor activity than would flow resistance studies of the type employed in assessing arteriolar constriction, since A'enous capacity is a more significant variable than is venous resistance to blood flow. 6 The following observations on splanchnic venous distensibility in anesthetized dogs demonstrate directly the constriction of venous elements as part of the response observed in the common pressor reflexes.From the Department of Physiology, Medical College of Georgia, Augusta, Ga.This investigation was supported by grunts from the National Heart Institute, U. S. P. II. S., and from the Life Insurance Medical Research .Fund.Received for publication April 26, 1954. METHODSAs described previously, 5 the physical characteristics of the splanchnic venous system demand rigorously controlled rates of distension and exact intervals between measurements if comparable distensibility determinations are to be obtained. This has been achieved in these experiments by injecting blood into the venous system of intestinal loops at the rate of 50 <:c. per minute by means of a motor-driven syringe, all injections being spaced accurately at JO-minute intervals for both control and experimental observations. The injection procedure described below required less than 30 seconds; for the remainder of the 10-minute interval between injections, perfusion of the loop was maintained by the dog at its existing arterial blood pressure.To preserve the sensitivity of cardiovascular reflexes, the dogs were anesthetized with chloralose following morphine sedation, employing 75 ing. of chloralose per kilogram for inducti...

The carotid sinus baroreceptor reflex in the pregnant rabbit

“…Heymans & Neil (1958) have stressed the contribution of venoconstriction to any rise in cardiac output. Though the degree of limb vein constriction when sinus pressure is reduced may be small (Browse, Donald & Shepherd, 1966;Bevegard & Shepherd, 1966;Brender & Webb-Peploe, 1969), venoconstriction in the splanchnic bed seems well-established (Alexander, 1954;Oberg, 1964;Brender & Webb-Peploe, 1969). Reports of structural alterations in the rabbit vasculature induced experimentally by steroids or as a result of pregnancy (Danforth, Manalo-Estrella & Buckingham, 1964), of increased venous distensibility produced in women by pregnancy or oestradiol (Goodrich & Wood, 1964, and of inhibition by oestradiol and progesterone of the response to electrical stimulation of strips of human venous smooth muscle (Barwin & McCalden, 1972), may indicate therefore that impaired venous contractility could contribute to the smaller responses which we have observed in the pregnant rabbits.…”

Studies on Starling's Law of the Heart. I. The Circulatory Response to Acute Hypervolemia and Its Modification by Ganglionic Blockade

“…One may speculate that during the "control" portion of this study and in experiments reported by others (2-4, 6, 7, 17), this expansion of blood volume increased intravascular pressure. The consequent stimulation of arterial (20) and systemic venous baroreceptors (21) could have resulted in a decrease in venous tone resulting in an increased capacity of the venous bed at any given transmural pressure (21)(22)(23)(24). This decrease in venous tone, in turn, could result in a tendency for the infused fluid to pool in the systemic venous bed, which would minimize any augmentation of the volume of blood in the heart and lungs (25).…”

“…It is postulated that as a consequence of the partial ganglionic blockade induced by Arfonad, venous tone diminished, the capacity of the venous bed increased and the central blood volume, cardiac output, and left ventricular work fell. However, it would appear likely from the observations of Alexander (22), that when venous tone diminishes, for any given increment in venous blood volume the transmural venous pressure rises to a greater extent than when the venous bed is initially partially constricted. This would permit a larger fraction of the infused blood to enter the thorax and cardiac output to rise significantly.…”

Role of the venous system in baroreceptor-mediated reflexes in man

“…One mneans by which such regulation can be achieved is through l)arorecel)tor-mediated reflexes, and substantial evidence is available to sul)port the contention that this system plays an important role in the regulation of the degree of Nrenocoinstriction. Thus, a number of investigations hav-e (lemolnstrated that a reduction in carotid sinus pressure produces venoconstriction in the dog (7)(8)(9)(10)(11)(12), and that carotid sinus hypertension prodluces venodilation (7,9,10). As a result of such studies, in their moiiograph Heymans and Neil wrote that "the most important change (luring reflex systemic hypertension of sino-aortic origin is Jrobably that of venoconstriction" (13).…”

“…The role of the baroreceptor system in regulating venous tone in man is also disputed. Thus, upright tilt, lower-body negative pressure, and drug-in-(luced hypotension have all been shown to be followed by venoconstriction ( 1 [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20]; on the other hand, several recent studies have shown that the venoconstriction after upright tilt and lowerbody negative pressure occurs irregularly and is generally slight and transient (21)(22)(23). It has been our experience in studies on conscious man that it is often extremely difficult to determine whether an increase in venous tone after an intervention which decreases baroreceptor activity is truly mediated by the baroreceptor system, or is the result of the well-known responsiveness of the venous system to many sensory and emotional stimuli.…”