Urinary bladder distension: its effects on carotid baroreceptor reflex left ventricular inotropic response in the dog

GABAergic control of micturition within the periaqueductal grey matter of the male rat

In urethane-anaesthetised rats continuous infusion of saline into the bladder (6 ml h⁻¹) evoked periodic sharp rises in intravesicular pressure accompanied by rhythmic bursting of external urethral sphincter (EUS) EMG and expulsion of urine from the urethral meatus. Microinjection of the GABA agonist muscimol (250 pmol) into the caudal ventrolateral periaqueductal grey (PAG), but not at other sites in the PAG, either depressed reflex voiding frequency (-60%, n = 7) and tonic EUS EMG activity (-38%, n = 6) or completely inhibited voiding (four sites). Microinjection of the GABA antagonist bicuculline (BIC; 1 nmol) into the same region, to reduce ongoing GABA tone, increased reflex voiding frequency (+467%, n = 16) and tonic activity in the EUS (+56%, n = 7) whilst bursting activity in the EUS became desynchronised. Although muscimol failed to change reflex micturition when microinjected into the dorsal caudal PAG, microinjection of BIC at these sites evoked pronounced autonomic arousal and increased reflex voiding frequency (+237%, n = 34). The results demonstrate that the functional integrity of synapses in the caudal ventrolateral PAG is essential to permit micturition. Transmission through the region is normally regulated by a tonic GABAergic inhibitory influence. In contrast, the functional integrity of the dorsal caudal PAG is not essential for reflex micturition. However, micturition may be initiated from this region via projections to the caudal ventrolateral PAG, as part of the behavioural response to psychological threat or other stressful stimuli.

Cardiovascular responses to carotid chemoreceptor stimulation in the dog: their modulation by urinary bladder distension

Respiratory, heart rate and hindlimb vascular responses were studied in response to increasing levels of stimulation of the carotid body chemoreceptors, together with an examination of the modulation of their effects by distension of the urinary bladder in the dog anaesthetized with a mixture of chloralose and urethane. The vascularly isolated carotid bifurcation regions were perfused with blood, stimulation of the carotid bodies being carried out by three different levels of hypoxic isocapnic blood (PO2 approximately 58, 40 and 22 mmHg) obtained from a donor animal. A vascularly isolated hindlimb was autoperfused at constant blood flow through its femoral artery. In spontaneously breathing animals, increasingly intense hypoxic stimulation of the carotid bodies caused a progressive augmentation of respiratory minute volume. Superimposition of distension of the bladder increased ventilation further, by the same amount during hypoxic as during normoxic blood perfusion of the chemoreceptors. Prevention of the effects of lung stretch afferent stimulation by artificial ventilation modified the heart rate and hindlimb vascular responses to excitation of the carotid bodies by revealing or accentuating the primary cardiovascular responses, bradycardia and vasoconstriction. In contrast, no such respiratory modulation was apparent in the cardiovascular responses to bladder distension. When, under conditions of artificial ventilation and in the absence of changes in the arterial baroreceptor input, the primary cardio-inhibitory and vasoconstrictor responses to carotid chemoreceptor stimulation predominated, the heart slowed progressively as the stimulus was increased. At the same time the cardio-accelerator effects of bladder distension progressively diminished, indicating an interaction between the cardiac reflex responses evoked by the two inputs. In contrast, the reflex vascular responses resulting from stimulation of the two inputs were additive, at least for PO2 levels of carotid body perfusate down to approximately 40 mmHg. In conclusion these experiments demonstrate the differential nature of the integration of respiratory and cardiovascular responses evoked by stimulation of the carotid chemoreceptors and bladder distension.

Bladder contractions alter inspiratory termination by superior laryngeal and intercostal nerve stimulation

Gradual distension of the urinary bladder evokes spontaneous bladder contractions (SBCs), which are associated with reduced inspiratory activity in the phrenic and other inspiratory motor nerves. We examined the influence of isovolumetric SBCs on the threshold for termination of phrenic inspiration by electrical stimulation of superior laryngeal and/or mid-thoracic intercostal nerves (ICN) in decerebrate, vagotomized, paralyzed, ventilated cats. Although SBCs reduced phrenic inspiratory activity, the threshold for inspiratory termination by nerve stimulation was increased. The results emphasize the complexity of the synaptic connections among brain stem neurons governing micturition and breathing.

Respiratory modulation of carotid and aortic body reflex left ventricular inotropic responses in the cat

1. The reflex changes in the inotropic state of the left ventricle, measured as the dP/dt max (maximum rate of change of pressure), occurring in response to selective stimulation of the carotid and aortic body chemoreceptors by sodium cyanide, were studied in the cat anaesthetized with a mixture of chloralose and urethane. 2. The animals were artificially ventilated with an open pneumothorax. The heart rate and mean arterial blood pressure were maintained constant. 3. With on-going central respiratory activity, stimulation of the carotid bodies caused an increase in respiratory movements. Variable changes in left ventricular dP/dt max occurred, the predominant response being an increase. The mean change was 8.3 +/- 2.9 % from a control value of 6850 +/- 450 mmHg s-1. Stimulation of the aortic bodies resulted in a smaller increase in respiration or no effect, but a significant increase occurred in left ventricular dP/dt max of 19.6 +/- 2.9 % from a control value of 6136 +/- 228 mmHg s-1. No significant changes in left ventricular end-diastolic pressure occurred in response to stimulation of either group of chemoreceptors. 4. Tests of chemoreceptor stimulations were repeated during temporary suppression of the secondary respiratory mechanisms: the central respiratory drive was suppressed reflexly by electrical stimulation of the central cut ends of both superior laryngeal nerves and lung stretch afferent activity was minimized by stopping artificial respiration. Carotid body stimulation again evoked variable responses, the predominant now being a reduction in left ventricular dP/dt max of 3.1 % from a control value of 5720 +/- 320 mmHg s-1, which was significantly different to that occurring during on-going spontaneous respiration. Aortic body stimulation caused an increase in left ventricular dP/dt max similar to the response during on-going spontaneous respiration. 5. The positive inotropic responses were mediated via the sympathetic nervous system, as indicated by their abolition as a result of intravenous injections of the beta-adrenoceptor blocking agent, propranolol. 6. It is concluded that the carotid bodies exert a small variable effect on left ventricular dP/dt max, the predominant positive inotropic response being due to the concomitant neurogenic effects of the increase in respiration. In contrast, the positive inotropic response to excitation of the aortic chemoreceptors is not respiratory modulated.

Reflex haemodynamic responses caused by distension of the uterus in anaesthetized pigs

The present study was undertaken in anaesthetized pigs to determine whether distension of the uterus reflexly affects the aortic blood pressure, heart rate, left ventricular inotropic state and the coronary circulation. Experiments were performed in 17 pigs anaesthetized with alpha-chloralose and artificially ventilated. Coronary blood flow was measured with an electromagnetic flowmeter positioned around the origin of the left circumflex coronary artery. The uterus was distended by injecting 20 ml warm Ringer solution into a balloon positioned within the uterus (mean transmural pressure of about 17 mmHg). Distension of the uterus without controlling any haemodynamic variable caused an increase in aortic blood pressure. When this response was prevented, an increase in heart rate was obtained in each animal. When the heart rate and blood pressure responses were prevented, the distension did not cause significant changes in the maximum rate of change of left ventricular pressure, but always caused a decrease in mean coronary blood flow. In five pigs, the increase in heart rate and the decrease in mean coronary blood flow were graded by step increments of distension. In six pigs, the haemodynamic responses to distension of the uterus were not affected by the administration of atropine. In 12 pigs, which included the six given atropine, the increase in heart rate was abolished by the administration of propranolol and the increase in aortic blood pressure and the decrease in mean coronary blood flow were abolished by the subsequent administration of phentolamine. In the remaining five pigs, the haemodynamic responses caused by uterine distension were abolished by the administration of bretylium tosylate. The present study showed that distension of the uterus in anaesthetized pigs primarily caused reflex increases in heart rate and aortic blood pressure and coronary vasoconstriction. These reflex responses were mediated by efferent sympathetic mechanisms.