Gillian P. Courtice

Effect of frequency and impulse pattern on the non-cholinergic cardiac response to vagal stimulation in the toad, Bufo marinus

In the toad, Bufo marinus, stimulation of the vagosympathetic trunk to the heart in the presence of cholinergic and adrenergic blockade results in cardiac slowing. This study investigates the importance of impulse pattern and frequency of neural stimulation in determining this cardiac response. When 360 stimuli were delivered to the heart either continuously at 3 Hz for 2 min, 4 Hz for 1.5 min or 6 Hz for 1 min, or in pulses at 6 Hz for 1 s every 2 s, over a 2 min period, there were no significant differences in the size of the chronotropic responses observed. However, when 360 stimuli were delivered in pulses at 6 Hz for 0.5 s every 1 s over 2 min, the resulting cardiac slowing was significantly greater than in response to the other stimulus regimens. In addition, the cardiac slowing in response to 8 Hz for 0.5 s every 1 s over 2 min was significantly greater than the response to 4 Hz continuously for 2 min. The results provide evidence to support the suggestion that the non-cholinergic, non-adrenergic cardiac response to stimulation of the vagosympathetic trunk is peptidergic in origin, and that the frequency of impulses is important in the gain of the response.

Excitation of the cardiac vagus by vasopressin in mammals.

In unanaesthetized sheep, the sensitivity of the baroreceptor‐cardio‐inhibitory reflex was greater when intravenous vasopressin was used to raise blood pressure, than when intravenous phenylephrine was used to raise blood pressure. This difference was still evident in animals in which beta‐adrenergic blockade had been carried out using propranolol. In the presence of combined beta‐adrenergic and muscarinic blockade, a direct negative chronotropic effect of intravenous vasopressin could not be demonstrated. It was concluded, therefore, that intravenous vasopressin enhanced cardiac vagal tone. This effect of vasopressin on efferent cardiac vagal tone was confirmed directly in anaesthetized dogs by recording from single cardiac vagal efferent fibres. Furthermore, recordings from single carotid sinus baroreceptor fibres did not demonstrate a direct action of vasopressin on the sensitivity of the baroreceptors. However, the pressor effect of vasopressin is associated with a greater increase in efferent cardiac vagal discharge than that seen when equipressor doses of phenylephrine are given, or when blood pressure is raised by a similar amount by inflation of an intra‐aortic balloon. Studies in isolated guinea‐pig atrial preparations and in anaesthetized rabbits and dogs, revealed no consistent peripheral action of vasopressin on the action of the vagus at the heart.

Effect of neuropeptide-Y and galanin on autonomic control of heart rate in the toad, Bufo marinus

The effects of neuropeptide-Y (NPY) and galanin (GAL) on the autonomic control of heart rate were investigated in the anaesthetised toad, Bufo marinus. Both vagosympathetic trunks were sectioned to prevent reflex changes in heart rate, and the cardiac responses to electrical stimulation of either the vagal or sympathetic fibres to the heart assessed. Intravenous, bolus doses of 10 or 20 micrograms (2 or 4 nmol) NPY and 5 or 10 micrograms (1.5 or 3 nmol) GAL caused pronounced pressor responses but small direct changes in heart rate. Pulse intervals measured after peptide administration were within 5% of control values. All doses of both peptides caused inhibition of action of the cardiac vagus nerves, the maximum inhibition observed in response to 20 micrograms NPY: mean 49.5 +/- 14% (SEM). No significant changes in cardiac sympathetic nerve action were observed. It is concluded that NPY and GAL have similar, important cardiovascular actions in the toad. Similarities between the responses of toads and mammals to NPY suggest a phylogenetic conservation of function for this peptide.

Vasoconstrictor effects of galanin and distribution of galanin containing fibres in three species of elasmobranch fish

Galanin is found in perivascular sympathetic neurons in a wide range of vertebrate species. In placental mammals, galanin has either no effect on blood pressure, or weak depressor effects, but in other vertebrates it has been shown to be a potent pressor agent. To investigate how extensive the vasoconstrictor effects of galanin may be in the vertebrates, the vascular effects of galanin were tested in two species of shark, Heterodontus portusjacksoni, and Hemiscyllium ocellatum, and a ray, Rhinobatos typus. Nerve fibres showing immunoreactivity to galanin were located surrounding gut blood vessels, but were absent from branchial efferent arteries in all three species. Intravenous injection of galanin caused a significant rise in caudal arterial blood pressure in H. portusjacksoni and H. ocellatum, but no change in R. typus. Contraction of segments of pancreatico-mesenteric artery were measured in an organ bath also. Galanin (10(-6) M) caused 21-38% of the maximum K+ induced contraction in all species, but no response in efferent branchial arteries from R. typus. In conclusion, in three elasmobranchs, a galanin-like peptide is present in perivascular nerve fibres, and galanin causes differential vasoconstriction in vascular beds. These data extend the number of vertebrate groups in which galanin has been shown to be a vasoconstrictor peptide.

Inhibition of cardiac vagal action by galanin but not neuropeptide Y in the brush-tailed possum Trichosurus vulpecula.

1. Stimulation of the right cardiac sympathetic nerve for 2 min at 16 Hz in the presence of either beta‐ or alpha‐ and beta‐adrenoceptor blockade evoked attenuation of cardiac vagal action in eight possums: 31.3 +/‐ 10.3% maximum inhibition of cardiac vagal action on prolonging pulse interval, with a time to half‐recovery of 4.9 +/‐ 1.1 min. 2. Intravenous injection of galanin (2‐3.5 nmol kg‐1) evoked similar inhibition of cardiac vagal action: 41.3 +/‐ 4.1% maximum inhibition of cardiac vagal action on pulse interval, with a time to half‐recovery of 13.4 +/‐ 2.3 min. 3. Intravenous injection of neuropeptide Y (NPY) at greater molar doses (6.5‐10 nmol kg‐1) caused no inhibition of cardiac vagal action. 4. The galanin injections caused a powerful pressor response: 57.1 +/‐ 4.9 mmHg increase in systolic blood pressure. NPY caused a smaller pressor response, despite administration of higher molar doses: 36.7 +/‐ 3.0 mmHg increase in systolic blood pressure. 5. In the possum, galanin but not NPY can mimic the effects of cardiac sympathetic nerve stimulation on vagal action. Galanin also causes large pressor effects.

Comparative vascular responses in elasmobranchs to different structures of neuropeptide Y and peptide YY

The vascular responses to neuropeptide Y (NPY) and peptide YY (PYY) were tested in several species of elasmobranchs to assess whether changes in sequence in these neuropeptides from elasmobranchs to mammals are associated with different physiological responses. NPY-like immunoreactivity was detected in the gut and in nerve fibres surrounding some, but not all, blood vessels of six species. Intravenous injection of dogfish, frog and human NPY in anaesthetised fish caused similar vasopressor effects in the three species tested, except human NPY which lowered blood pressure in one of the three. Dogfish NPY and PYY were equipotent pressor agents in two species, but PYY was significantly more potent than NPY in one species. NPY and PYY both contracted isolated gut arteries from three species, but had no effect on isolated efferent arteries tested. In conclusion, differential vascular responses in elasmobranchs are not associated with changes in NPY sequence across vertebrates, but may be with changes in PYY in some species.

Selective regional vasoconstriction underlying pressor effects of galanin in anaesthetized possums compared with cats.

1. Intravenous administration of porcine galanin (5 nmol kg‐1) caused a rise in mean blood pressure in the brush‐tailed possum, Trichosurus vulpecula, from 58 +/‐ 1.6 to 106 +/‐ 1.6 mmHg. This effect is in contrast to the cat, in which no significant change in blood pressure was recorded in response to galanin (88 +/‐ 2.3 vs. 86 +/‐ 2.4 mmHg). 2. Cardiac output and regional blood flow distribution were assessed by distribution of radioactive microspheres in four anaesthetized possums and four cats, before and after administration of galanin. 3. Cardiac output was 289.8 +/‐ 14.0 ml min‐1 in the cat and 189.9 +/‐ 25.5 ml min‐1 in the possum. Galanin administration did not significantly change cardiac output in either species. 4. In the possum, galanin administration caused large increases in resistance to flow in the spleen, gut, adrenal glands, kidney, skin and carcass. The largest increase was in the kidneys, where renal blood flow fell to 6% of control levels. 5. In the cat, changes in resistance were small. Small increases in resistance to flow in muscle and carcass were offset by small decreases in resistance in the lungs and kidneys. 6. The results suggest that the pressor effect of galanin in the possum is the result of direct vasoconstrictor action in several vascular beds, in contrast to the cat, in which such effects are few and weak.

Vagal stimulation and somatostatin potentiate cardiac vagal action in the toad, Bufo marinus

Cholinergic postganglionic neurones of the cardiac vagus in the toad, Bufo marinus, have been shown to contain the peptide somatostatin (SOM), which causes direct negative inotropic and chronotropic effects on the heart. In anaesthetised toads, high frequency stimulation (10 Hz) of cardiac vagus nerves results in prolonged cardiac slowing and potentiation of the cardiac slowing measured in response to a train of vagal stimuli at low frequency. Intravenous administration of the tetradecapeptide form of SOM also results in prolonged cardiac slowing and potentiation of cardiac vagal action. Effects on heart rate of small bolus doses of acetylcholine (ACh) were unaltered by administration of SOM, at the same time as cardiac vagal slowing was enhanced. It is suggested that SOM is released from vagal nerve endings by high frequency stimulation and enhances cardiac vagal action by a presynaptic mechanism.

Cardiac vagal action during hypoxia in adult and fetal sheep

The effects of hypoxia on the potential for the vagus to slow the heart, and on resting heart rate, were compared in anesthetized, vagotomized adult and fetal sheep, and in a chronically catheterized fetus in utero. In adults, the action of the cardiac vagus was potentiated at and below an arterial pO2 in the range 13-27 mm Hg. In contrast, in the fetus and the neonate, vagal action was not potentiated as pO2 fell through this range to 10-12 mm Hg. Below 10-12 mm Hg baseline heart rate fell markedly, and the effect of the cardiac vagus on heart rate was diminished, but its effect on pulse interval was not consistently changed. It is concluded that potentiation of vagal action during hypoxia occurs in adult but not fetal sheep and the bradycardia seen in the fetus during severe hypoxia is probably due to direct myocardial depression.

Desensitisation of the cardiac response to somatostatin in isolated toad atria but not in whole toads

The cardiac slowing measured in response to repeated application of somatostatin (SOM) was compared in an isolated toad sinus venosus/atrial preparation and in a whole, anaesthetised toad, Bufo marinus. Repeated doses of 0.5-1 x 10(-7) M SOM in the organ bath progressively reduced the cardiac response to 25% or less of the response to the first dose given. When preparations were allowed a recovery period of 50-80 min, the response recovered to 51.0 +/- 3.7% of the control. In contrast, the cardiac chronotropic response in anaesthetised toads to repeated intravenous injections of 2 micrograms or 1.2 nmol SOM over 3 h was not changed. Prolonged exposure to SOM (0.1-0.25 mumol/kg/h infusion), for up to 4 h in anaesthetised toads caused no significant reduction either in the cardiac responses to a bolus dose of SOM (0.6 nmol) or nerve released SOM (3 Hz for 2 min) in the presence of atropine. These opposing results suggest that the half-life of a peptide and the method of exposing receptors to a peptide are important factors in receptor desensitisation.