Differential effects of morphine on forearm blood flow: Attenuation of sympathetic control of the cutaneous circulation

Morphine sulfate (MS) induces net peripheral arteriolar dilation due to reduction in alpha adrenergic control. In order to determine the relative effects of this action on cutaneous and skeletal muscle circulations, forearm blood flow (FBF) and mean arterial pressure (BP) were measured and forearm vascular resistance (FVR) was calculated in 7 normal male subjects before (C) and after the intravenous administration of 15 mg of MS. In each subject, FBF was determined plethysmographically in an intact forearm and in a forearm subjected to epinephrine iontophoresis, a technique which specifically arrests only the cutaneous circulation. Cutaneous FBF was calculated from the difference between the FBF of the intact and the iontophoresed arms. No change in BP occurred with MS (C: 74; MS: 74 mm Hg, p > 0.5). Intact FBF increased with MS (C: 1.8; MS:3.9 ml/min · 100 ml, p < 0.05) while FVR was reduced (C:49.3; MS:27.9 mm Hg/ml/min · l00 ml, p < 0.01). Skeletal muscle FBF and FVR did not change with MS (FBF, C: 1.2, MS: 1.3, p > 0.5; FVR, C:74.1; MS: 77.3, p > 0.5). Cutaneous FBF increased with MS while cutaneous FVR was reduced (FBF, C:0.7, MS: 2.4, p < 0.05; FVR, C:93.3, MS:43.0, p < 0.025). These data indicate that clinical doses of morphine cause net limb vasodilation limited to the cutaneous circulatory bed.