ROLE OF SYMPATHETIC-NERVES IN THE VASCULAR EFFECTS OF LOCAL TEMPERATURE IN HUMAN FOREARM SKIN

The role of adrenergic nerve function in the cutaneous vascular respon se to changes in local skin temperature in the human forearm was exami ned using three protocols: 1) blocking release of norepinephrine presy naptically by local iontophoresis of bretylium (BT), 2) altering backg round adrenergic tone by changing whole body skin temperature, and 3) blocking cutaneous nerves by proximal infiltration of local anesthetic . Forearm skin blood flow was measured by laser-Doppler flowmetry (LDF ) and cutaneous vascular conductance (CVC) was calculated as LDF/blood pressure. In protocol 1, local cooling (29-degrees-C) elicited a rapi d and sustained fall in CVC at control sites (-43 +/- 8%) in contrast to a biphasic response at BT-treated sites, consisting of an initial v asodilation followed by a vasoconstriction (percent change CVC = 28 +/ - 13 and -34 +/- 18, respectively). Local warming (39-degrees-C) incre ased CVC at control and at BT-treated sites by 331 +/- 46 and 139 +/- 31 %, respectively. In protocol 2, at a neutral, cool, or warm whole b ody skin temperature, local cooling (29-degrees-C) elicited similar re ductions in CVC (-34 +/- 8, -29 +/- 5, and -30 +/- 4%, respectively), and local warming (38-degrees-C) produced similar increases in CVC (89 +/- 15, 85 +/- 21, and 74 +/- 22%, respectively). In protocol 3, bloc king cutaneous nerves did not affect either the vasoconstriction produ ced by local cooling to 29-degrees-C (CVC = -29 +/- 12%, control; cuta neous nerve block = -41 +/- 5%) or the vasodilator response to local w arming to 39-degrees-C (increase in CVC, 320 +/- 69%, control; cutaneo us nerve block = 311 +/- 42%). Our results suggest that 1) intact adre nergic nerve terminals and norepinephrine release, but not sympathetic nerve activity per se, are required for the immediate vasoconstrictor response to local cooling; 2) norepinephrine release can be locally i nitiated, perhaps through an axon reflex; 3) responses to prolonged lo cal cooling involve mostly nonadrenergic mechanisms; and 4) the majori ty of the vasodilator response to local warming does not require an in tact adrenergic system.