Adrenergic innervation of cat iris sphincter

Isometric tension changes of the iris sphincter of the cat were investigated in vitro. Electrical stimulation resulted in contraction followed by relaxation. Elimination of the cholinergic component by chronic parasympathetic denervation or by atropine, or both, markedly reduced electrically induced contraction, but had little influence on relaxation. Norepinephrine produced contraction, relaxation, or slight relaxation followed by contraction. Isoproterenol elicited virtually pure relaxation. The actions of dichloroisoproterenol and phentolamine indicated that relaxation induced electrically or by drug was due mainly to activation of beta receptors and contraction was due to activation of alpha receptors. Proper choice of electrical stimulus parameters (high versus low stimulus rate) allowed activation predominantly of relaxing beta adrenergic, or of contractile alpha adrenergic mechanisms, respectively. These studies also confirm earlier observations that electrical stimuli act indirectly, i.e., via excitation of nerve terminals and release of transmitter, and that the sphincter muscle is not directly excitable by electrical shocks.

Alteration of vascular responses to endotoxin by adrenergic blockade

The effects of the adrenergic blocking agent phenoxybenzamine (Dibenzyline) on the vascular responses to injected histamine and endotoxin were studied in dogs utilizing a "venous return" preparation. A marked antihistaminic action of Dibenzyline was demonstrated in the hepatic portal system. Less effective blocking was seen in the arterial segment. The effectiveness of Dibenzyline for preventing increases in portal venous pressure and pooling of blood were similar with both histamine and endotoxin administration. These data show a basic similarity between the early vascular responses to histamine and endotoxin in the hepatic-portal system. A major dissimilarity between the vascular effects of histamine and endotoxin was the absence of an early drop in total peripheral resistance after endotoxin. Beneficial effects of Dibenzyline after endotoxin, during the first 100 min, could not be demonstrated in animals whose cardiac output was unsupported. The action of Dibenzyline alone is apparently incapable of reversing the detrimental effects of endotoxin.