Cholinergic nerves in the rat portal vein

Pharmacological characterization and autoradiographic localization of dopamine receptors in the portal vein

1. Dopamine (DA) and DA receptor agonists exert a variety of effects on the cardiovascular system through interaction with specific DA receptors, including decreases in blood pressure and heart rate. 2. The decrease in blood pressure is due primarily to arterial vasodilation. This phenomenon is due to the stimulation of both postjunctional (D1-like or DA1) and prejunctional (D2-like or DA2) receptors causing respectively relaxation of arterial smooth muscle and decrease of the sympathetic vasoconstriction tone. 3. In view of the lack of detailed information on the existence of DA receptors in venous tissue, we have analysed D1-like and D2-like receptors in the rat portal vein using radioligand binding techniques associated with light microscope autoradiography. 4. No D1-like receptors were demonstrated in sections of the rat portal vein, whereas the D2-like receptor ligand, [3H]-spiroperidol, was bound to sections of the vein in a manner consistent with the labelling of D2-like sites. Anatomically, D2-like sites were located within the tunica adventitia, including the adventitia-media border, and in the endothelium. 5. These findings suggest the existence of D2-like but not D1-like receptor sites in the rat portal vein. D2-like sites of the tunica adventitia are probably prejunctional and involved in the modulation of sympathetic outflow. The functional significance of endothelial D2-like sites, if any, should be clarified in future studies.

Comparison of VIP and beta 2-adrenoceptor-induced relaxations in the circular and longitudinal smooth muscle layers of the rat portal vein

The relative importance of VIP in reduction of vascular tone was studied in circular and longitudinal preparations of the VIP-innervated rat portal vein. Exogenous VIP inhibited the methoxamine-evoked contractures in the atropine-blocked preparations with a lower potency in the inner, circular (pD2 = 6.4 +/- 0.5, n = 6) than in the outer, longitudinal layer (pD2 = 7.7 +/- 0.1, n = 6). VIP was also a less efficient relaxant (intrinsic activity (alpha) = 0.60 +/- 0.16, n = 6) of the inner than of the outer layer (alpha = 1.00). The selective (salbutamol) and the non-selective (isoproterenol) beta 2-agonists completely relaxed the methoxamine contractures in both layers and the potency (isoproterenol) was higher in the inner (pD2 = 6.39 +/- 0.32, n = 6) than in the outer layer (pD2 = 5.67 +/- 0.34, n = 6). Plasma from the portal-mesenteric vein of anaesthetized, fasting rats contained 0.036 nM VIP (median, n = 17), that is, several orders of magnitude lower than the range of VIP concentrations relaxing the methoxamine contracted vein preparations via VIP receptors of the apamin-blockable category. The results support the hypothesis that alpha 1-adrenoceptor-induced contractions in the circular layer are predominately relaxed via beta 2-adrenoceptors while relaxation of the outer layer may occur via VIP receptors, probably activated by local release of the neuropeptide.

Immunohistochemical evidence for mast cells containing 5-hydroxytryptamine in rat portal vein

Serotonin was localized to mast cells in the adventitia of the rat portal vein by indirect immunohistochemistry. The mastocytes where preferentially localized to a region delimited by the pyloric and splenic veins. Since neither 48/80 nor reaginic antibody induced a significant change in the intrinsic spontaneous activity of the portal vein it would appear that the mast cells are not involved in a direct vasomotor function. It is suggested that amines released from the mastocytes could regulate blood flow in the vasa vasorum and/or have a role associated with the sensory functions displayed by this vein.

Effect of methionine-enkephalin on the spontaneous electrical and mechanical activity of the smooth muscle of the rat portal vein

In the longitudinal smooth muscle of the isolated rat portal vein, methionine-enkephalin (Met-enkephalin) increased the spontaneous contraction with a concentration as low as 10(-8)M. When the membrane activity was recorded using a microelectrode, Met-enkephalin enhanced the spike burst activity but without any effect on the resting membrane potential. Naloxone, phentolamine, atropine and reserpine pre-treatment did not inhibit the excitatory effect of Met-enkephalin on the spontaneous contraction. These results suggest that the excitatory effect of Met-enkephalin on the mechanisms involved in the automaticity may be a direct action on smooth muscle or relate to presynaptic action on a non-adrenergic non-cholinergic system.

Actions of neurotransmitters and peptides on longitudinal and circular muscle of the rat portal vein

The reactivity of longitudinal and circular muscle of the rat portal vein to noradrenaline, acetylcholine, 5-hydroxytryptamine, substance P, angiotensin II and neurotensin was compared. Longitudinal muscle was prepared as longitudinal strips and circular muscle was studied as transversally cut rings. Longitudinal muscle was more sensitive than circular muscle to acetylcholine, 5-hydroxytryptamine and angiotensin II, whereas both muscle layers were equally sensitive to noradrenaline and substance P. Circular muscle was generally unresponsive to neurotensin. Differential sensitivity of longitudinal and circular muscle layers suggests that these two muscle layers do not necessarily operate in unison.