P. Hadházy

Effect of duration and frequency of stimulation on the presynaptic inhibition by α adrenoceptor stimulation of the adrenergic transmission

SummaryIt had been shown previously that in rat vas deferens prostaglandins fail to influence neurochemical transmission. Therefore, this preparation provides a possibility for studying the transmitter release in a preparation in which any interaction with prostaglandins was in all likelihood excluded.1.The inhibitory effect of α-adrenoceptor stimulation by either noradrenaline, BAY-1470 or clonidine on the response of rat vas deferens and guinea-pig atria to field stimulation depended on the rate of stimulation and on the length of train applied. The higher the frequency and the longer the train the smaller was the inhibition.2.Direct evidence for the presynaptic inhibitory effect of α-adrenoceptor stimulation on transmitter release has also been provided by measuring the release of 3H-noradrenaline; there was a reduction at 2 Hz stimulation, but not at 50 Hz.

Characteristics of coronary endothelial dysfunction in experimental diabetes

OBJECTIVE To study the influence of diabetes on the endothelium-dependent vasodilation in the coronary arterial bed. METHODS The effects of acetylcholine (ACh 2-36 pmol.kg-1; 18 nmol.1(-1)-9.8 mumol.1(-1); 0.1-10 mumol.1(-1), L-arginine (1 mmol.1(-1) and sodium nitroprusside (1 nmol.1(-1)-100 mumol.1(-1)) were measured on coronary conductivity, vascular tone and cGMP release (RIA) in healthy and diabetic dogs. RESULTS ACh-mediated (in cumulative intra-arterial infusion) increase in coronary conductivity was reduced (P < 0.01) in the diabetic dogs in vivo, whereas no increase in cGMP release was observed in isolated diabetic coronaries (P < 0.05) which could not be enhanced by L-arginine (P < 0.05). Inhibition of cyclo-oxygenase after 20 min further impaired (P < 0.01) responsiveness to ACh in vivo and diminished the ACh response in isolated coronary strips of the diabetic dogs, but not in those of the controls. Relaxation in response to sodium nitroprusside was not altered by diabetes. CONCLUSIONS Diminished vasodilation in diabetes is due to a defect in endothelial nitric oxide production and action. Vasodilating prostanoids do not sufficiently compensate this defect.

Differential contractile responsiveness of isolated rabbit arteries from different vascular beds to cyclooxygenase inhibitors and PGI2

The actions of three, structurally unrelated cyclooxygenase inhibitors and PGI2 on the contractile responses to electrical stimulation and to noradrenaline of strips of rabbit coeliac, ear, pulmonary, carotid, femoral arteries and the aorta were studied. Indomethacin (3 mumol/l), suprofen (0.8 mumol/l) and meclofenamic acid (0.2 mumol/l) potentiated the adrenergically induced contractions of coeliac arteries by 126-165%. The contractile responses of ear arteries were increased by these three substances by 87-91%, and the responses of pulmonary arteries by 26-33%. All of these changes were statistically significant. Prostacyclin produced a dose-related inhibition of the stimulation-induced contractions of the coeliac, ear and pulmonary arteries; its IC50 values were 10.4, 212 and 433 nmol/l, respectively. In contrast to effects in the above arteries, the evoked contractions of aortic and carotid strips were not affected by any of the prostaglandin synthesis inhibitors used; the responses of femoral vessels were reduced by all of the inhibitors (by 13-23%; P less than 0.05). Low concentrations of PGI2 did not affect the evoked contractions of aortic, carotid and femoral strips whereas higher concentrations increased baseline tone and potentiated contractions. The results indicate that there are considerable difference in the sensitivities of the arteries from different vascular beds to inhibitors of cyclooxygenase and to prostacyclin.

The effect of number of stimuli and rate of stimulation on the inhibition by PGE1 of adrenergic transmission.

Abstract An inverse correlation was found between the number of pulses and sympathetic blockade by PGE 1 (10 −8 g/ml) in guinea-pig isolated field stimulated atria and vas deferens. The higher the number of pulses applied at a given frequency the smaller was the inhibition by PGE 1 of neuroeffector transmission. At 100 shocks using 10 Hz stimulation, PGE 1 no longer decreased the height of twitches of the guinea-pig vas deferens, however, the velocity of contractions was decreased. Sympathetic transmission in the rabbit jejunum was blocked by a somewhat higher dose of PGE 1 (3 × 10 −8 g/ml). In this organ an increase in pulse number produced no detectable change in PGE 1 -induced blockad. At a temperature of 37°C the vas deferens of the guinea-pig was highly sensitive to PGE 1 , whereas that of the rat was virtually insensitive. On cooling the guinea-pig vas deferens to a temperature of 20°C the sensitivity to PGE 1 decreased and the shape of the shock number-effect curve became similar to that observed in that rat vas deferens. It is suggested, that the different shape of the curve may be caused by reduced release and/or effect of an inhibitory substance, probably prostaglandin. The biological responsesobtained with racemic PGE 1 were qualitatively identical with those elicited by natural PGE 1 the latter being about twice as potent.

Inhibition of adrenergic neurotransmission by prostaglandin E1 (PGE1) in the rabbit ear artery

Abstract Isolated central ear arteries of rabbits were perfused with Krebs solution at a constant flow rate of 2–7 ml/min. Pressure changes to periarterial nerve stimulation (1, 3, 10 and 30 Hz; 5–32 pulses) and to noradrenaline were measured in the presence and absence of prostaglandin E 1 (PGE 1 ). This substance in concentrations of 7 × 10 −9 – 4.5 × 10 −7 produced a dose-dependent inhibition of constrictor responses to nerve stimulation. The degree of inhibitory action of PGE 1 became progressively less as the frequency or length of stimulation was increased. Prostaglandin E 1 (1.1 × 10 −7 M) failed to affect the constrictor responses to noradrenaline and it markedly reduced the stimulation-evoked release of radioactivity from arteries incubated with [ 3 ]noradrenaline. The results indicate that PGE 1 inhibits the vasoconstrictor responses of rabbit ear artery to nerve stimulation by reducing the release of the adrenergic transmitter.

The effects of PGE1 on responses to cardiac vagus nerve stimulation and acetylcholine release

Abstract PGE 1 , 2.5 × 10 −9 to 5 × 10 −9 g/ml, reduced the negative chronotropic responses to vagal stimulation and to acetylcholine in isolated guinea-pig atria. Neither ganglionic transmission nor axonal impulse conduction were depressed by the compound in anaesthetized cats. PGE 1 did not affect acetylcholine release from the parasympathetic nerve terminals of the guinea-pig ileum. These findings are in disagreement with the view that PGE 1 modulates parasympathetic transmission by reducing the release of the cholinergic transmitter. It reduces vagal bradycardia by antagonizing the effect of acetylcholine on the atrial cells. This ‘anticholinergic’ action may be related to the cyclic AMP system.

Effects of indomethacin and prostaglandins I2 and E2 on the tone of human isolated mesenteric arteries.

The actions of indomethacin (IND), PGE2 and PGI2 were studied on the tone of isolated mesenteric arteries obtained from operated patients. The cyclooxygenase inhibitors IND (3 mumol/l) and suprofen (0.58 mumol/l) increased the resting tone and potentiated the contractile responses to electrical stimulation and noradrenaline. Low concentrations of PGE2 (0.7-5.6 X 10(-9) mol/l) decreased the baseline tone and reduced the stimulation-evoked contractions whereas higher concentrations (from 5.7 X 10(-8) mol/l) increased the tone of vessels. PGI2 (0.7-10.8 X 10(-9) mol/l) also relaxed IND-treated arteries but, in contrast to PGE2, it did not produce contraction even at a concentration of 10(-6) mol/l. Prostacyclin reduced the tone evoked and sustained by a high concentration of PGE2 or PGF2 alpha, the IC50 values being 46.2 or 7.9 X 10(-9) mol/l, respectively. The contractile responses to electrical stimulation and to noradrenaline were also inhibited by PGI2 (IC50 5.6 and 6.8 X 10(-9) mol/l, respectively). These results suggest that the smooth muscle cells of human mesenteric arteries are just as sensitive to IND, PGE2 and PGI2 as are those from laboratory animals. Our observations may be of clinical importance.

Altered Responsiveness of Diabetic Dog Renal Arteries to Acetylcholine and Phenylephrine: Role of Endothelium

The mechanical responses to acetylcholine (ACh), sodium nitroprusside and phenylephrine (PE) were determined in normal and diabetic dog renal arterial strips with and without endothelium. Experimental diabetes increased the sensitivity, IC50 = (2.9 +/- 0.4) X 10(-8) mol/l in normal and (9.6 +/- 1.5) X 10(-9) mol/l in diabetic (p less than 0.01, n = 6) to ACh of endothelium intact renal arterial strips without influencing the maximum relaxation induced by this agonist. In all intact vessels PE produced contractions of equal magnitude. Removal of the endothelium completely abolished the relaxant ability of ACh, and caused a slight increase in the contractile response of both diabetic and normal strips to PE. The maximum contractile force generated by the denuded diabetic vessels in response to PE was significantly (p less than 0.01) greater than the maximum tension produced by the denuded nondiabetic arteries. The sensitivity of the tissues to PE was, however, not modified by either diabetes or endothelium removal. The direct relaxant sodium nitroprusside elicited a similar degree of relaxation in the two groups of arteries. Cyclooxygenase blockade had no effect on either the relaxation or contractile responses of any of the preparations. These findings suggest that short-term diabetes makes dog renal arteries supersensitive to ACh and hyperreactive to PE.

Effects of hypoxia and adrenergic stimulation induced alterations in PGI2 synthesis by diabetic coronary arteries

Before the onset of histologically detectable alterations in diabetic arteries, a considerable decrease in vasodilatory potential is seen. While analyzing this phenomenon, the role of altered PGI2 synthesis in rings of coronary arteries from metabolically healthy and alloxan-diabetic dogs was measured by radioimmunoassay during baseline, under the influence of phenylephrine (100 mumol/L), and during hypoxia with or without the presence of the alpha adrenergic blocker phentolamine (5 mumol/L). Basal levels of PGI2 synthetized by healthy and diabetic coronaries were no different (7.9 +/- 2.1 and 6.4 +/- 1.4 pg/mg vessel). Phenylephrine potentiated PGI2 synthesis in controls (150 +/- 22%), while it proved to be ineffective in the diabetic animals (98 +/- 6%). Under hypoxic conditions, PGI2 production of healthy coronaries (152 +/- 24%) increased, while that in the diabetic ones (82 +/- 7%) decreased (p less than 0.01). In the presence of phentolamine no difference could be detected between the two groups. Given all these data, the decreased ability of the diabetic coronaries to vasodilate develops due to diminished PGI2 production, presumably controlled by adrenergic mechanisms. Furthermore, the more severe outcome of ischaemic heart disease in diabetes mellitus might be explained by the lack of an enhanced coronary PGI2 synthesis under hypoxic conditions.

Feedback control of arterial smooth muscle tone: the role of prostacyclin

1 Indomethacin potentiated noradrenaline (NA)‐induced contractions of rabbit isolated mesenteric arteries. Mechanical removal of the endothelium did not influence its potentiating effect. 2 Prostacyclin (PGI2) synthesis was stimulated by NA (0.1‐1.0 μmol 1−1) and inhibited by indomethacin in a concentration‐dependent manner. 3 There was a positive correlation between the indomethacin‐induced inhibition of PGI2 formation and the indomethacin‐evoked potentiation of contractile responses to NA. 4 These results suggest that endogenous PGI2 plays a part in the feedback control of vascular smooth muscle tone.