Effect of temperature and baroreceptor stimulation on reflex venomotor responses

To investigate the interaction of thermal reflexes and baroreflexes in the control of the peripheral veins, we studied in supine humans the effects of lower body negative pressure (LBNP) and neck suction (NS) on forearm veins at ambient temperatures (T(a)) of 18, 28, and 37°C. Forearm venous volume (FVV)-venous pressure (FVP) relations (forearm venous capacitance) on six subjects showed an increase from 18 through 28 to 37°C (P<0.001). Heart rate increased (P<0.001) and forearm venous capacitance decreased (P<0.001) in proportion to the level of LBNP applied from 20 to 50 Torr at all T(a). At 50 Torr LBNP, FVV at 30 cmH₂O, FVP decreased from control values of 2.5, 3.8, and 4.4 to 1.6, 2.7, and 3.4 ml/100 ml at 18, 28, and 37° C, respectively. We also studied venomotor responses using the occluded limb technique. Although LBNP caused venoconstriction, NS applied either alone or during LBNP produced no change in venomotor tone. Therefore we concluded that carotid baroreceptors play little role in reflex venomotor adjustments. Since changes in mean arterial and pulse pressures during LBNP did not account for the observed venomotor responses, we concluded that low-pressure baroreceptors initiate significant venoconstrictor reflexes over a wide range of T(a).