Search

The contractile recoverability of isolated portal veins made hypodynamic by removal of glucose from the perfusion medium by diverse exogenous substrates was studied. It was found that glucose, mannose, pyruvate, lactate, acetate, and butyrate readily restored contraction. The effects of anoxia and metabolic inhibitors were also investigated. Both anoxia and dinitrophenol abolished spontaneous activity within 10 min. Fluoroacetate markedly depressed contractility (-60.9 +/- 2.8% at 10 min), the reduction being essentially similar in medium with or without glucose. 2-Deoxy-D-glucose (2-DG) in glucose medium reduced contractility only slightly whereas in the absence of glucose it drastically inhibited contraction (-76% at 10 min and -97% at 40 min). Intracellular ATP was significantly reduced by anoxia and fluoroacetate in medium with or without glucose and by 2-DG only in substrate-free medium. However, ATP depletion showed no clear correlation with the extent of contractile depression. These findings indicate a strong dependence for mechanical activity in this vein on aerobic metabolic pathways. But, from the results with 2-DG, it is suggested that an adequate glucose metabolism might be of importance for functional processes other than contraction itself.

The levels of isometric developed tension (IDT) in portal veins from starved and diabetic rats in glucose medium were of the same magnitude but significantly reduced compared with veins from normal rats. Glucose removal led to contractile depression, more marked in starved veins (-74%; p < 0.001 vs. diabetics and normals). Addition of glucose, pyruvate, acetate, lactate, or butyrate counteracted this depression in all groups but in diabetics butyrate significantly stimulated IDT over controls whereas in starved veins lactate overcame IDT reduction only partially. Added insulin significantly enhanced contractile recovery with glucose in normal and diabetic preparations but was ineffective in starved veins. Glycogen content was significantly lower in diabetics; however, in no group did changes occur in incubated veins as compared with unincubated ones. Glycerol production was significantly higher in diabetic veins. It would appear that, although experimental diabetes induces a shift to fatty acid utilization, aerobic glycolysis and mitochondrial respiration are somehow preserved as reflected by the IDT recovery with pyruvate or lactate.

Oxygen uptake and isometric developed tension (IDT) of isolated rat portal vein were simultaneously measured after glucose removal, following the readmission of metabolic substrates and addition of 2-deoxy-D-glucose (2-DG) or insulin. In control conditions, basal O2 uptake was 0.128 +/- 0.004 microliter O2/mg dry weight/min (n = 50). After 40 min of incubation in a glucose-free medium, IDT and O2 uptake decayed to a similar degree. Addition of 11 mM glucose, 5 mM pyruvate, acetate, lactate or butyrate to hypodynamic veins, elicited differential recoveries in the mechanical and metabolic parameters, pyruvate and acetate being the most effective. Insulin (0.1 U/ml) added 5 min prior to readmission of 11 mM glucose, increased the restoration of IDT without any effect on oxygen uptake. 2-DG 5.5 mM, when added in glucose-free medium, abolished IDT but only depressed the cellular respiration by 38% at 20 min; these effects were decreased in presence of glucose (11 mM). These results suggest that the oxygen uptake of this vessel depends, among other factors, on the exogenous substrate offered. They show also the relationship between peak of developed isometric force and metabolic activity as well as the relevance of glycolytic and oxidative pathways in this tissue.

The studies concerned the effects of epinephrine, norepinephrine, phenylephrine, acetylcholine and K+ ions, in doses inducing maximal contraction on oxygen consumption of isolated rat portal veins. All the agonists tested stimulated the rate of O2 uptake, whereas high K+ did not. After specific blockade of alpha-adrenoceptors, the effects of epinephrine and norepinephrine on O2 consumption were not affected, whilst those of phenylephrine were abolished, and beta-adrenoceptor blockade suppressed the metabolic effect of epinephrine and norepinephrine. Acetylcholine was the agonist that most elevated O2 consumption; atropine blocked both contractile and metabolic effects of this drug. It appears that there is a clear dissociation between contractile and metabolic actions of some pharmacological agents, and a question is posed concerning phenylephrine and acetylcholine effects. Results with K+ suggest that different metabolic pathways might be involved in the activation and maintenance of drug-induced contraction in smooth muscle.

The present study investigates the effects of Sodium Nitroprusside (SNP) on isometric developed tension (IDT) and tonic component of K+ induced contracture of rat portal vein. The availability of activator calcium for the contraction was modified by changing the ionic composition of the Krebs Ringer bicarbonate medium (KRB) or by using pharmacological agents such as nifedipine or KCl. In control conditions (KRB media) magnitude of IDT was 10.61 +/- 0.56 mN (n = 13). Addition of increasing concentrations of SNP (non cumulative) diminished IDT in a dose related manner. Tonic component of 60 mM KCl evoked contracture decayed 27.04 +/- 10.9% (n = 6) after 10(-4)M SNP incubation. Relaxant effect of SNP was more pronounced in the presence of nifedipine 10(-10)M (72.35 +/- 7.5% of inhibition, n = 6). The sensitivity of the preparation to SNP was strongly reduced in conditions of diminished Ca+2 efflux (slow sodium media = KRB sucrose). Experiments here presented support the hypothesis that the vasodilator effect of SNP could be attributed to an enhanced Ca+2 efflux from the vessel.

Effects of calcium antagonists on spontaneous mechanical activity and pharmacological reactivity of isolated rat portal vein were investigated. Prenilamine (PREN) 2 X 10(-4) M, verapamil (VER) 2 X 10(-5) M and nifedipine (NIF) 3 X 10(-8) M abolished spontaneous phasic contractions. In veins incubated in calcium-free medium plus PREN, VER or NIF, readmission of calcium restored mechanical activity up to similar control values when [Ca2+]0 reached 4.88 mM. Contractures evoked by epinephrine (EPI) 2.5 X 10(-5) M or KCl 90 mM were diminished in a similar degree by all three antagonists, its pharmacological potency being NIF greater than VER greater than PREN. Tonic component of contracture was significantly more depressed than phasic. Contracture evoked by methoxamine 10(-4) M was impaired by prazosin 10(-8) M as well as by NIF 3 X 10(-10) M. The alpha1 antagonist also blocked EPI contractile effects. Comparative studies were performed in isolated rat tail ventral artery; contractions evoked by KCl 120 mM or EPI 5 X 10(-5) M or methoxamine 10(-4) M were differentially affected by calcium omission or by NIF 3 X 10(-8) M. Results support the hypothesis that calcium antagonists act mainly by interfering with calcium influx across cell membrane without selective action upon voltage or receptor operated channels.

Conflicting reports exist about the effects of diabetes on vascular function. In the present study we investigated (1) the influences of diverse stages of diabetes on mechanical activity and pharmacological reactivity of portal vein and tail ventral artery isolated from male rats 7, 21 or 30 days after alloxan injection (150-180 mg/kg) and (2) the effects of in vitro or in vivo insulin treatment. Various parameters were used to assess the diabetic state (serum glucose levels, body weight, percentage of glycosylated hemoglobin and glucosuria). Isometric developed tension of portal vein from control rats was 10.86 +/- 0.41 mN (n = 54), and was enhanced significantly in diabetics (+56% at 21 days and +45% at 30 days; p less than 0.001 vs. controls). When challenged with noradrenaline, portal veins from diabetics exhibited a greater contractility and lower reactivity (as reflected by EC50 values). The magnitude of responses to KCl remained similar to those obtained in controls, but nonetheless the reactivity seems to be higher. Tail ventral artery from diabetics also exhibits a greater contractility in response to noradrenaline with no significant changes in EC50 values. The results demonstrate that diabetes affects mechanical performance of the vascular smooth muscle in a differential manner depending on the stage of the endocrinopathy and on the types of vessel studied. These modifications were not avoided by insulin treatment.