Regional differences in sympathetic neurotransmission to cutaneous arteries in the guinea-pig isolated ear

The effects of sympathetic nerve stimulation on different cutaneous arteries were examined in arteries isolated from guinea-pig ears, by measuring membrane potential changes in smooth muscle cells in response to electrical field stimulation. Resting membrane potential (RMP) was similar in proximal (main ear artery) and distal (3rd or 4th branch order) cutaneous arteries (mean -71 mV). Single stimuli evoked excitatory junction potentials (EJPs) in all arteries. The EJPs in proximal arteries were twice the amplitude, and the time constant of EJP decay was almost half the value, compared with distal cutaneous arteries. EJP amplitude was reduced by > 90% by suramin (30 microM) or alpha,beta,methylene-ATP (alpha,beta,m-ATP)(1 microM) in all proximal, and most distal arteries. Residual responses in distal arteries were resistant to tetrodotoxin. The N-type calcium channel blocker, omega-conotoxin GVIA (30 nM), reduced EJP amplitude by 70-100% in both proximal and distal arteries. Successive EJPs evoked by trains of stimuli at 1 to 5 Hz were depressed in amplitude in proximal arteries, but showed facilitation in distal arteries. EJP depression in proximal arteries was reversed to facilitation by the alpha2-adrenoceptor antagonist, yohimbine (30 nM). Trains of stimuli delivered at 10-20 Hz produced summation of EJPs and active membrane responses in 30% of proximal arteries. Active responses were never detected in distal arteries. Slow depolarizations following the EJPs were detected in most arteries after trains of stimuli, and were abolished by prazosin (0.3 microM) or omega-conotoxin GVIA (30 nM). The density of the perivascular plexus of axons innervating proximal arteries, demonstrated with catecholamine fluorescence histochemistry, was twice that in distal cutaneous arteries. These regional differences in sympathetic neurotransmission suggest that cutaneous vasoconstriction in response to thermoregulatory stimuli, which occurs predominantly in distal cutaneous segments, is likely to be qualitatively different from cutaneous vasoconstriction of proximal arteries in response to other physiological stimuli.