REGIONAL DIFFERENCES IN SYMPATHETIC NEUROTRANSMISSION TO CUTANEOUS ARTERIES IN THE GUINEA-PIG ISOLATED EAR

The effects of sympathetic nerve stimulation on different cutaneous ar teries were examined in arteries isolated from guinea-pig ears, by mea suring membrane potential changes in smooth muscle cells in response t o electrical field stimulation. Resting membrane potential (RMP) was s imilar in proximal (main ear artery) and distal (3rd or 4th branch ord er) cutaneous arteries (mean -71 mV). Single stimuli evoked excitatory junction potentials (EJPs) in all arteries. The EJPs in proximal arte ries were twice the amplitude, and the time constant of EJP decay was almost half the value, compared with distal cutaneous arteries. EJP am plitude was reduced by > 90% by suramin (30 mu M) or alpha,beta,methyl ene-ATP (alpha,beta,m-ATP) (1 mu M) in all proximal, and most distal a rteries. Residual responses in distal arteries were resistant to tetro dotoxin. The N-type calcium channel blocker, omega-conotoxin GVIA (30 nM), reduced EJP amplitude by 70-100% in both proximal and distal arte ries. Successive EJPs evoked by trains of stimuli at 1 to 5 Hz were de pressed in amplitude in proximal arteries, but showed facilitation in distal arteries. EJP depression in proximal arteries was reversed to f acilitation by the alpha(2)-adrenoceptor antagonist, yohimbine (30 nM) . Trains of stimuli delivered at 10-20 Hz produced summation of EJPs a nd active membrane responses in 30% of proximal arteries. Active respo nses were never detected in distal arteries. Slow depolarizations foll owing the EJPs were detected in most arteries after trains of stimuli, and were abolished by prazosin (0.3 mu M) or omega-conotoxin GVIA (30 nM). The density of the perivascular plexus of axons innervating prox imal arteries, demonstrated with catecholamine fluorescence histochemi stry, was twice that in distal cutaneous arteries. These regional diff erences in sympathetic neurotransmission suggest that cutaneous vasoco nstriction in response to thermoregulatory stimuli, which occurs predo minantly in distal cutaneous segments, is likely to be qualitatively d ifferent from cutaneous vasoconstriction of proximal arteries in respo nse to other physiological stimuli. (C) 1998 Elsevier Science B.V. All rights reserved.