Forearm vascular and neuroendocrine responses to graded water immersion inhumans

The hypothesis that graded expansion of central blood volume by water immer sion to the xiphoid process and neck would elicit a graded decrease in fore arm vascular resistance was tested. Central venous pressure increased (P < 0.05) by 4.2 +/- 0.4 mmHg (mean +/- SEM) during xiphoid immersion and by 10 .4 +/- 0.5 mmHg during neck immersion. Plasma noradrenaline was gradually s uppressed (P < 0.05) by 62 +/- 8 and 104 +/- 11 pg mL(-1) during xiphoid an d neck immersion, respectively, indicating a graded suppression of sympathe tic nervous activity. Plasma concentrations of arginine vasopressin were su ppressed by 1.5 +/- 0.5 pg mL(-1) (P < 0.05) during xiphoid immersion and b y 2.0 +/- 0.5 pg mL(-1) during neck immersion (P < 0.05 vs, xiphoid immersi on). Forearm subcutaneous Vascular resistance decreased to the same extent by 26 +/- 9 and 28 +/- 4% (P < 0.05), respectively, during both immersion p rocedures, whereas forearm skeletal muscle vascular resistance declined onl y during neck immersion by 27 +/- 6% (P < 0.05). in conclusion, graded cent ral brood Volume expansion initialed a graded decrease in sympathetic nervo us activity and AVP-release. Changes in forearm subcutaneous vascular resis tance, however, were not related to the gradual withdrawal of the sympathet ic and neuroendocrine vasoconstrictor activity. Forearm skeletal muscle vas odilatation exhibited a more graded response with a detectable decrease onl y during immersion to the neck. Therefore, the forearm subcutaneous vasodil ator response reaches saturation at a lower degree of central volume expans ion than that;of forearm skeletal muscle.