On the mechanism of L-dopa-induced postural hypotension in the cat

Comparative aspects of central cardiovascular control with special reference to adrenergic mechanisms

1. The central nervous system probably serves as an overall cardiovascular regulatory centre in all vertebrates. 2. Electrical stimulation of the brain elicits haemodynamic effects in all vertebrate classes, but the quality of the responses may differ between different classes in some cases. 3. In the most "primitive" classes of vertebrates, i.e. fishes, the circulatory regulation may be exerted via mechanisms somewhat different from those in other vertebrates. 4. Amphibians, reptiles, birds and mammals display a similar cardiovascular response pattern to administration of drugs such as clonidine and L-DOPA.

Reduction in blood pressure, sympathetic nerve discharge and centrally evoked pressor responses by methysergide in anesthetized cats

Methysergide (1 and 3 mg/kg i.v.) caused dose-dependent reductions in blood pressure and heart rate of anesthetized cats. In addition, sympathetic nerve discharges of the postganglionic renal nerve were also markedly reduced by the same doses of drug. Pressor responses to electrical stimulation of the diencephalon were inhibited by 3 mg/kg but not 1 mg/kg of methysergide. These results suggest that methysergide acts to decrease blood pressure by a centrally mediated reduction in sympathetic nervous outflow and, at higher doses, can additionally prevent pressor changes caused by electrical activation of suprabulbar central structures. These results are in agreement with previous reports that methysergide has little or no peripheral effects on the cardiovascular system.

Cardiovascular responses to acute hypoxia in dogs pretreated with benserazide and L-DOPA

In the control dogs, hypoxia induced different hemodynamic modifications (i.e. an increase of arterial blood pressure, heart rate, cardiac output, regional blood flow) which seem to be related to a release of epinephrine. Pretreatment with benserazide (20 mg/kg i.v.) and L-Dopa (50 mg/kg i.p.) either lessened or completely suppressed any sign of sympathetic stimulation. These results suggest that chemoreceptors are centrally inhibited as had been previously proved for baroreceptors.

Centrally mediated hypotension and bradycardia by methysergide in anesthetized dogs

In anesthetized dogs, methysergide (1 and 3 mg/kg i.v.) caused reductions in systolic and diastolic blood pressure, heart rate, left ventricular pressure and peripheral resistance. Caardiac output was unchanged because of an increase in stroke volume. Methysergide exhibited no alpha-receptor, ganglion, or adrenergic neuron-blocking properties nor did it have marked direct vasocilator action. The BCO, but not the orthostatic, reflex was severely inhibited by the drug, evidence for a central inhibitory action. Atropine, vagotomy or carotid sinus debuffering had little or no effect on the hypotension and bradycardia produced by methysergide, whereas guanethidine pretreatment essentially abolished these effects. Direct intracerebronventricular administration of small doses of methysergide (0.2 mg/kg) caused significant hypotension and bradycardia. It is concluded that methysergide causes centrally mediated hypotension and bradycardia, the mechanism of which is not clearly understood.

Gastrin release by bombesin in the dog

1 The intravenous infusion of bombesin produced in intact dogs and, more strikingly in dogs provided with gastric fistulae a sharp increase in plasma levels of immunoreactive gastrin and at the same time a stimulation of gastric acid secretion. Gastrin response was correlated with the dose of bombesin from approximately 0.1 mug kg(-1) h(-1) (threshold) to 1 mug kg(-1) h(-1) (maximum gastrin release).2 Atropine and metiamide reduced or inhibited gastric acid secretion stimulated by bombesin, but did not affect the rise in gastrin levels.3 Acidification of the whole stomach or of a perfused antral pouch caused a reduced or delayed response to bombesin. However, the inhibitory effect of acidification could be surmounted by prolonging the duration of bombesin infusion.4 Antrectomy greatly reduced the rise in gastrin levels and the increase in acid gastric secretion produced by bombesin, but left unaffected the gastric secretagogue effect of pentagastrin.5 It is concluded that bombesin is a potent releaser of gastrin from the antral mucosa.6 The possible influence of the renal effects evoked by bombesin in the dog on the gastrin response to the polypeptide is discussed.

An analysis of the peripheral effects of l-DOPA on autonomic nerve function

1 The ability of intravenous L-DOPA to block sympathetic and parsympathetic nerves has been studied in cats and dogs pretreated with a monoamine oxidase inhibitor.2 L-DOPA inhibited positive chronotropic and pressor responses to dimethylphenylpiperazinium (DMPP) and McN-A-343 in dogs, and contractions of the nictitating membrane produced by these ganglion stimulants in cats.3 Responses of the cat nictitating membrane to preganglionic stimulation were inhibited by L-DOPA to a greater extent than those to postganglionic stimulation of the cervical sympathetic chain.4 In dogs, L-DOPA had no vagolytic action, but depressed vasoconstrictor responses elicited in the perfused hind-limb by electrical stimulation of the lumbar sympathetic chain.5 The degree of lumbar sympathetic chain inhibition correlated with the pressor response following L-DOPA, and both effects were prevented by prior decarboxylase inhibition.6 These results suggest that the decarboxylation products of L-DOPA do not impair parasympathetic nerve activity but depress sympathetic nerve function predominantly by inhibiting both muscarinic and nicotinic sites of sympathetic ganglia.

The cardiovascular effects of L-dopa in the pithed rat

1 L-DOPA (1-2 mg i.v.) in the pithed rat reduced the arterial and increased the venous pressure responses to noradrenaline.2 Infusions of dopamine (4-8 mug kg(-1) min(-1)) and noradrenaline (500 ng kg(-1) min(-1)) also reduced the pressor responses to noradrenaline. The pressor response did not recover after stopping dopamine infusions, but it usually did so after stopping noradrenaline infusions.3 The effect of L-DOPA on the response to noradrenaline was prevented by the prior injection of the dopa decarboxylase inhibitor NSD 1024.4 The prior injection of the dopamine-beta-hydroxylase inhibitor diethyldithiocarbamate, only partially prevented the effect of L-DOPA on pressor responses to noradrenaline.5 The perfusion of noradrenaline (400 ng kg(-1) min(-1)) together with Krebs solution (10 ml/min) through an organ bath containing an isolated aortic strip, depressed the response of the strip to doses of noradrenaline after the infusion was stopped. Infusions of dopamine (0.5-8.0 mug kg(-1) min(-1)) had a similar effect. Fifteen minutes after adding L-DOPA (0.5 mg) to the bath and 10 min after washing it out, the response to noradrenaline was depressed in three out of four experiments.6 Infusions of noradrenaline (1 mug kg(-1) min(-1)) in an isolated heart perfused by Langendorf's method blocked the response to injected noradrenaline whilst perfusion was in progress. Infusions of dopamine (4-8 mug kg(-1) min(-1)) or of L-DOPA (200 mug kg(-1) min(-1)) did not have this effect.7 It is concluded that the effect of L-DOPA on pressor responses to noradrenaline in the pithed rat are mediated by its conversion to dopamine and noradrenaline.