A QUANTITATIVE MEASURE OF THE VASOMOTOR TONE IN THE HINDLIMB MUSCLES OF THE DOG

Abstract

The hindlimb muscles of anesthetized dogs were perfused with defibrinated blood at constant pressure from a pump-lung circulation. The sciatic nerve to the muscles was left intact. At intervals the nerve was reversibly blocked by cooling and the changes of blood flow measured at different perfusion pressures. The blood perfusion could be interrupted by periods of Ringer perfusion. With the blood vessels unconstricted (nerve blocked) the relations between the pressure and the flow of blood or of Ringer''s soln. were similar to those found by Whittaker and Winton (1933) in the isolated hindlimb. During vasoconstriction, the pressure at which the pressure-flow curve of blood becomes approx. linear was increased; below this pressure the slope diminished and the curve approached the origin. The extrapolated intercept of the linear part of the curve increased with increasing vasoconstriction. The Ringer pressure-flow curve was a straight line which intercepted the pressure axis at or near the origin; its slope was diminished by vasoconstriction but its intercept was unaffected. The apparent relative viscosity of the blood (ratio of Ringer flow to blood flow at constant pressure) was increased during vasoconstriction. The amt. of increase varied in different muscles and in the same muscles with the degree of vasoconstriction and with the pressure. Extreme values for blood of normal corpuscular concentration were 2-8. At normal pressures the change of apparent viscosity accounted for about 1/3 of the total change of resistance to blood flow caused by vasoconstriction. For any constant degree of vasoconstriction the ratio of the Ringer flows at constant pressure in the unconstricted and in the constricted blood vessels was approx. equal to the ratio of the slopes of the pressure-flow curves of blood in the unconstricted and in the constricted vessels, both slopes being measured over the linear parts of their characteristics. The evidence suggests that this ratio is independent of the viscosity and is a measure of the change in the av. dimensions of the blood vessels. In the innervated prepn. it is proposed as a measure of the vasomotor tone. Its value has varied from 1.0 (no vasomotor tone) to about 3.5. Reasons are given for supposing that the measure is quantitatively comparable in the muscles of different expts.