Reduction of vagal pressor reflexes by neurohypophyseal peptides and related compounds

Cardiovascular effects of felypressin

Cardiovascular effects of felypressin (FEL) were studied in Wistar rats. Heart rate and mean arterial pressure measurements were taken in awake rats treated with vasopressin (AVP), FEL, or epinephrine (EPI). Each group received either an intravenous (IV) or an intracerebroventricular V1 receptor antagonist, saline, area postrema removal, or sham surgery. Analysis of variance and Student-Newman-Keuls (P < .05) were applied. Felypressin and AVP induced a pressor effect, and bradycardia was inhibited by IV V1 antagonist. Intracerebroventricular V1 antagonist and area postrema removal enhanced their pressor effects. Epinephrine induced a higher pressor effect and a similar bradycardia that was not affected by the treatments. It was concluded that FEL depends on V1 receptors to induce pressor and bradycardic effects, and that it produces a high relationship between bradycardia and mean arterial pressure variation depending on area postrema and central V1 receptors. These effects are potentially less harmful to the cardiovascular system than the effects of EPI.

The hemodynamic effects of intrathecal oxytocin in normal dogs

OBJECTIVE

To evaluate the hemodynamic effects produced by intrathecal administration of oxytocin in healthy isoflurane-anesthetized dogs.

STUDY DESIGN

Prospective single-dose trial.

ANIMAL POPULATION

Six healthy purpose-bred adult dogs weighing between 7.3 and 14.5 kg.

METHODS

Dogs were anesthetized with isoflurane and instrumented. Oxytocin at a dosage of 1.6 microg/kg was administered intrathecally at the cisternal space at time 0. Hemodynamic data were recorded immediately before and at 1, 5, 15, 30, and 60 minutes after oxytocin administration. Statistical analysis included an analysis of variance (ANOVA) for repeated measures over time. A P < .05 was considered significant.

RESULTS

Baseline values +/- standard error of the mean for heart rate, mean arterial pressure, central venous pressure, cardiac output, systemic vascular resistance, mean pulmonary arterial pressure, pulmonary arterial occlusion pressure, and pulmonary vascular resistance were 101 +/- 11 beats/minute, 76 +/- 7 mm Hg, 4 +/- 4 mm Hg, 1.9 +/- 0.7 L/min, 3834 +/- 2556 dynes x sec/cm5, 14 +/- 3 mm Hg, 4 +/- 2 mm Hg, and 430 +/- 201 dynes x sec/cm5, respectively. Variations from the baseline values were seen in all parameters after intrathecal oxytocin administration, but no statistically significant differences were found.

CONCLUSION

The intrathecal injection of 1.6 microg/kg of oxytocin is associated with minimal hemodynamic effects during isoflurane anesthesia.

CLINICAL RELEVANCE

This study revealed no clinically significant deleterious effects from the intrathecal administration of oxytocin, and investigations into its use as a perioperative analgesic are therefore warranted.

Effect of losartan on afferent nerve stimulation

The present study was undertaken to investigate the effects of losartan, a non-peptide angiotensin II subtype 1 (AT1) receptor antagonist, on both the pressor responses elicited by stimulation of afferent vagal nociceptive fibres and the involvement of the sympathetic nervous system (evaluated by plasma levels of noradrenaline and its co-neurotransmitter neuropeptide Y) in dogs. Electrical stimulation of the afferent fibres of the vagus (1, 5, 10 and 20 Hz) elicited a frequency-dependent increase in blood pressure and heart rate. Plasma noradrenaline levels only increased after stimulation at frequencies of 10 and 20 Hz. Plasma neuropeptide Y levels did not change. Losartan (10 mg/kg i.v.) induced both a decrease in resting blood pressure and an increase in basal plasma levels of noradrenaline and neuropeptide Y. Losartan failed to modify the magnitude of the electrically-evoked pressor and positive chronotropic responses. The angiotensin AT1 receptor antagonist elicited a fall in plasma noradrenaline values after a 1 Hz stimulation and abolished the increase in plasma noradrenaline levels induced by the 10 (but not 20) Hz stimulation. The data suggest that angiotensin AT1 receptors are not directly involved in acute pressor responses induced by stimulation of afferent vagal fibres. Moreover, the results show that, besides its sympatho-inhibitory effect, losartan can exert a sympatho-excitatory action as shown by the increase in the plasma levels of both noradrenaline and its coneurotransmitter, neuropeptide Y.

A study of alpha 1- and alpha 2-adrenoceptor responsiveness in diabetic insipidus dogs

The present study was performed to investigate the participation of circulating vasopressin in alpha-adrenoceptor responsiveness. Thus, we compared the pressor responses induced by selective alpha 1-or alpha 2-adrenoceptor stimulation in two groups of conscious dogs: a) normal animals and b) animals with surgically-induced diabetes insipidus. In addition, platelet alpha 2-adrenoceptors labelled with (3H)RX821002 were compared in the two groups. The pressor response to alpha 1-adrenoceptor stimulation [ie successive doses of noradrenaline (0.5, 1, 2, 4 micrograms/kg i.v.) after propranolol (1 mg/kg i.v.) plus yohimbine (0.5 mg/kg i.v.)] was significantly (P < 0.05) less pronounced in diabetic insipidus than in normal dogs. In contrast, the magnitude of the pressor effects of alpha 2-adrenoceptor stimulation [ie noradrenaline after propranolol plus prazosin (1 mg/kg i.v.)] was the same in the two groups of animals. Bmax and Kd values for (3H)RX821002 binding on platelets were similar in diabetic insipidus and normal dogs. This study shows that alpha 1- (but not alpha 2-) adrenoceptor responsiveness is decreased in diabetic insipidus suggesting the involvement of vasopressin in the mechanisms of the vascular alpha 1-adrenoceptor pressor response.

Vasopressin and oxytocin gene expression in the supraoptic and paraventricular nucleus of the spontaneously hypertensive rat (SHR) during development of hypertension

To study the regulation of hypothalamic vasopressin (VP) and oxytocin (OT) gene expression in relation to the development of hypertension, levels of VP mRNA and OT mRNA were determined in spontaneously hypertensive rats (SHR). Differences in VP and OT mRNA content of the supraoptic nucleus (SON) and paraventricular nucleus (PVN) of 4- and 10-week-old SHR and Wistar-Kyoto controls (WKY) were quantitated by dot-blot and Northern blot analysis. VP and OT pituitary content and VP plasma levels were measured by radioimmunoassays. VP mRNA levels were approximately 2-fold and 3-fold higher in the SON and PVN of 4-week-old SHR, respectively, as compared to controls. The OT mRNA levels were approximately 35% lower in both nuclei of the SHR. There was no difference in VP and OT pituitary content between 4-week-old SHR and WKY, but VP plasma levels were higher in SHR. In the 10-week-old SHR VP mRNA levels were still approximately 30-40% higher and the OT mRNA levels were approximately 40% lower in both nuclei when compared to age-matched WKY. Pituitary VP and OT contents were respectively 1.5-fold higher and 20% lower in the 10-week-old SHR than in 10-week-old WKY. VP plasma levels were still elevated in the SHR. The data indicate that in the hypothalamo-neurohypophyseal system of the SHR the VP system is in a higher state of activity, while the OT system is lower in activity.