Influence of basal perfusion pressure on vasoconstrictor and vasodilator responses induced sympathetically and reflexly in pump-perfused rat hindquarters.
The hindquarters of Wistar rats, under pentobarbital anesthesia, were vascularly isolated and perfused at a constant flow rate (6.25 ml/min) with autologous blood. Basal perfusion pressure varied from 60 to 140 mm Hg during the experimental procedure and among individual rats. The magnitude of the constrictor effect of i.a. norepinephrine was virtually constant and independent of the basal pressure. I.A. injection of acetylcholine and histamine; i.v. injection of norepinephrine; and stimulation of the lumbar sympathetic nerves with pulses of 0.03 msec or less, produced dilator responses in the perfused extremities. Stimulation with pulses of 0.1 msec or more resulted in a constrictor response followed by a dilator response. These dilator responses increased linearly and the constrictor responses decreased linearly as the basal pressure increased. Thus, the law of initial value of resistance held true for all dilator responses tested and for the constrictor responses evoked by stimulation of the lumbar sympathetic nerves. This result permitted normalization of these responses to predicted values at a reference perfusion pressure (100 mm Hg) by statistical analysis. The dilator response elicited by stimulation with shorter pulses and the constrictor response elicited by longer pulses had the same time course, which suggests that the constrictor response is probably compounded by a synchronous dilator mechanism. Hexamethonium, guanethidine and prazocin abolished the dilator responses elicited by neural stimulation and those induced by i.v. norepinephrine. These results indicate that the responsiveness of pump-perfused rat hindquarters varies with the existing perfusion pressure in a linear fashion according to the law of initial value of resistance except in the case of i.a. norepinephrine. Both the electrically- and reflexly-evoked dilator responses require intact ganglionic and alpha-adrenergic transmission.
D000109:Acetylcholine / Q000494:pharmacology; D000818:Animals; D004558:Electric Stimulation; D006145:Guanethidine / Q000494:pharmacology; D006584:Hexamethonium Compounds / Q000494:pharmacology; D006614:Hindlimb / Q000098:blood supply; D006632:Histamine / Q000494:pharmacology; D008297:Male; D009638:Norepinephrine / Q000494:pharmacology; D010477:Perfusion; D011224:Prazosin / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains; D012018:Reflex / Q000502:physiology; D012039:Regional Blood Flow / Q000187:drug effects*; D013564:Sympathetic Nervous System / Q000502:physiology*; D014661:Vasoconstriction / Q000187:drug effects*; D014664:Vasodilation / Q000187:drug effects*
D006584:Hexamethonium Compounds; D006632:Histamine; D000109:Acetylcholine; D009638:Norepinephrine; D011224:Prazosin; D006145:Guanethidine