P. Zapata

Effects of dopamine on carotid chemo- and baroreceptors in vitro.

1. The effects of dopamine on the sensory discharges originating from arterial chemo‐ and baroreceptors were studied in vitro using carotid bodies or sinuses excised from anaesthetized cats and superfused with Lockes solution. 2. Intrastream injections of dopamine 10–200 mug produced a transient depression of the frequency of chemoreceptor discharges. This effect was observed in response to the first injection in eighteen out of twenty preparations. 3. The inhibitory effect of dopamine can counteract partially or totally the excitation of chemoreceptors evoked by simultaneous application of acetylcholine or cyanide. 4. This inhibitory effect of dopamine is reduced or abolished by pretreatment with dopaminergic (Spiroperidol) or alpha‐adrenergic (Dibenamine) blockers. 5. In response to repeated injections of dopamine applied at short intervals, the inhibitory effect is replaced by a biphasic effect (early inhibition followed by late excitation), a late and long‐lasting excitation or no changes in chemoreceptor activity. The late excitatory effects of dopamine are not blocked by dopaminergic or alpha‐adrenergic blockers. 6. Noradrenaline does not affect the chemoreceptor activity of the superfused carotid body. DL‐DOPA induces only a late and long‐lasting excitatory effect. 7. In carotid sinus preparations, dopamine induces a weak but long‐lasting increase in the frequency of baroreceptor discharges. 8. It is concluded that dopamine may play a modulatory role in the generation of chemoreceptor activity through local regulatory processes.

Chemical, electron microscopic and physiological observations on the role of catecholamines in the carotid body

Abstract Chemical determinations revealed large amounts of dopamine, noradrenaline and adrenaline, in the carotid body of the cat. The cathecholamine (CA) content of normally innervated and chronically synmpathectomized carotid bodies was essentially the same. It did not change by vigorous and prolonged carotid body stimulation either in situ or in vitro . High doses of reserpine reduced appreciably the noradrenaline content of the carotid body; the contents of dopamine and adrenaline also were reduced but to a lesser degree. Electron microscopy shoed the presence of numerous dense-cored vesicles in the glomus cells of the carotid body; these vesicles occurred only rarely in the capsular cells and in the carotid nerve terminals. No detectable change in the vesicular content, appearance or distribution was induced by prolonge dhypoxia, reserpinization, adrenalectomy or chronic section of the carotid nerve or sympathetic supply. Acute and chronic reserpinization of cats did not change the sensitivity and reactivity of carotid bodies (either in situ or in vitro to ACh, anoxic or asphyxic stimulation. In vitro , high doses of adrenaline, noradrenaline, dopamine, dl -DOPA and tyramine failed to produced chemoreceptor excitation or marked depression of the chemosensory discharges. The lack of effect of CA on chemoreceptor activity was not changed by reserpinization or inactivation of monaminoxidase with nialamide. Prolonged superfusion with dischloroisoproterenol, a β-adrenergic blocking agent, reduced chemoreceptor discharges by inducing nerve block. It is concluded that CA contained in the carotid body do not play a significant role in the generation of the chemosensory discharges.

Effects of dopamine analogues and antagonists on carotid body chemosensors in situ.

1. The effects of dopamine, its analogues and antagonists on the chemosensory discharges originating from carotid bodies in situ were studied in anaesthetized cats. 2. Intracarotid (I.C.) injections of 100 ng or more of dopamine produced transient depression of the frequency of carotid nerve chemosensory discharges. Short term (1‐5 sec) complete inhibition was usually elicited by 2 microgram dopamine. 3. I.V. injections of dopamine also produced inhibition of chemosensory discharges, an effect observed with doses which were still too low to produce changes in systemic arterial pressure. Half‐maximal inhibition (ID50) of chemoreceptors was elicited with a mean dose of 84 ng.kg‐1. 4. I.C. and I.V. injections of apomorphine and amantadine also produced transient inhibition of chemosensory activity. Higher doses of these analogues of dopamine were needed to produce this effect, and the resulting inhibition usually did not silence the nerve discharges. Apomorphine inhibition was slightly more prolonged than that with dopamine. 5. Large doses of amphetamine and tyramine, inducers of dopamine release, did not produce inhibition of chemosensory discharges. 6. The effects of two classes of dopamine antagonists were tested. Dose‐response curves for dopamine and apomorphine inhibition were displaced to the right by administration of phenothiazines (chlorpromazine and perphenazine) and butyrophenones (haloperidol and spiroperidol). In animals treated with perphenazine or spiroperidol, dopamine became a stimulator of chemoreceptor activity. 7. It is suggested that dopamine present in carotid body may operate as a modulator of chemosensory activity.

The release of acetylcholine from carotid body tissues. Further study on the effects of acetylcholine and cholinergic blocking agents on the chemosensory discharge.

1. Both carotid bodies were removed from cats and placed in a small Perspex channel through which Locke solution was allowed to flow under a layer of paraffin oil. Stimulation of the upstream (‘donor’) organ elicited an increased sensory discharge in the downstream (‘detector’) preparation (Loewi effect).

Axon regeneration following a lesion of the carotid nerve: Electrophysiological and ultrastructural observations

Carotid nerves of the cat were crushed and allowed to regenerate in order to study the properties of regerating fibers and the role of carotid body parenchymal cells (glomus or type I, and sustentacular or type II) in the transduction of chemosensory activity. Such activity is reinitiated 6 days after the nerves are crushed close (1-2 mm) to the carotid body. The process of recovery is delayed when a crush is made at successively greater distances (5-6 and 10-12 mm) from the carotid body. Ultrastructural studies show that the reappearance of nerve endings on the glomus-sustentacular cell complex coincides in time with the onset of chemosensory activity. The regenerated nerve endings increase in size and number and appear normal by 48 days. Some barosensory activity can be elicited 6 days after a nerve crush close to the carotid sinus, but rhythmic barosensory discharges only occur after the 21st day when myelinated axons reappear in the carotid sinus adventitia. Results suggest that recovery of chemosensory function depends on the reestablishment of apposition between regenerating carotid nerve fibers and parenchymal cells of the carotid body.

Selective activation of carotid nerve fibers by acetylcholine applied to the cat petrosal ganglion in vitro

The petrosal ganglion innervates carotid body chemoreceptors through the carotid (sinus) nerve. These primary sensory neurons are activated by transmitters released from receptor (glomus) cells, acetylcholine (ACh) having been proposed as one of the transmitters involved in this process. Since the perikarya of primary sensory neurons share several properties with peripheral sensory endings, we studied the electrical responses of the carotid nerve and glossopharyngeal branch to ACh locally applied to the cat petrosal ganglion superfused in vitro. Ganglionar applications of AChCl (1 microg-1 mg) generated bursts of action potentials conducted along the carotid nerve, while only a few spikes were exceptionally recorded from the glossopharyngeal branch in response to the largest doses. Carotid nerve responses to ACh were dose-dependent, the higher doses inducing transient desensitization. Application of nicotine to the petrosal ganglion also evoked dose-dependent excitatory responses in the carotid nerve. Responses to ACh were reversibly antagonized by adding hexamethonium to the superfusate, more intense and prolonged block of ACh responses being produced by mecamylamine. Ganglionar applications of gamma-amino butyric acid and serotonin, in doses of up to 5 mg, did not induce firing of action potentials in any of the branches of the glossopharyngeal nerve. Our results indicate that petrosal ganglion neurons projecting through the carotid nerve are selectively activated by ACh acting on nicotinic ACh receptors located in the somata of these neurons. Thus, cholinosensitivity would be shared by the membranes of peripheral endings and perikarya of primary sensory neurons involved in arterial chemoreception.

Effects of dopaminergic blockade upon carotid chemosensory activity and its hypoxia-induced excitation

The effects of domperidone, antagonist of D2 receptors, on arterial chemoreceptor activity were studied in spontaneously breathing and pentobarbitone anesthetized cats, in which recordings of chemosensory impulse activity were obtained simultaneously from both cut carotid (sinus) nerves. Intravenous injections of domperidone 50 micrograms/kg produced a maintained increase in the basal frequency of chemosensory discharges, after which hyperoxic tests (breathing 100% O2 for 30 s) evoked larger falls in the rate of chemosensory impulses. Chemosensory responses evoked by hypoxic hypoxia (100% N2 tests) and by cytotoxic hypoxia (i.v. injections of NaCN) reached higher impulse rates after domperidone treatment. The effects of domperidone reveal that a resting release of dopamine from glomus cells maintains a low level of basal chemosensory activity under normoxic conditions. Domperidone turns off such restraining dopaminergic control and enhances the transient chemosensory responses to hypoxic stimuli. Present data support a modulatory role for dopamine within the chemoreceptor process, but not its participation as excitatory transmitter between glomus cells and sensory nerve endings.

Pharmacology of pH effects on carotid body chemoreceptors in vitro

1. The carotid body and the carotid nerve were removed from anaesthetized cats and placed in a small Perspex channel through which Locke solution (at various pH values and usually equilibrated with 50% O2 in N2) was allowed to flow. The glomus was immersed in the flowing solution while the nerve was lifted into oil covering the saline. Sensory discharges were recorded from the nerve and their frequency was used as an index of receptor activity. At times, a small segment of carotid artery, containing pressoreceptor endings, was removed together with the glomus. In this case, pressoreceptor discharges were recorded from the nerve.

Respiratory effects of dopamine-induced inhibition of chemosensory inflow

In pentobarbitone-anesthetized cats, intracarotid injections of dopamin (DA) 0.05--20 micrograms produced transient ventilatory depression, enhanced by section of the contralateral carotid nerve and abolished by section of the ipsilateral one. I.v. injections of DA 0.02--2 micrograms-kg-1 also induced transient hypoventilation; this effect was abolished by bilateral section of the carotid nerves. Slow i.v. infusion of DA 10 micrograms-kg-1-min-1 elicited initially a pronounced hypoventilation followed by a steady-state of mild ventilatory depression; these changes were absent after bilateral carotid neurotomy. Recordings from carotid nerves showed that DA-induced decreases of chemosensory activity to 50% of its control did not modify ventilation, while chemosensory arrests transiently depressed ventilation to 40--75% of its control level. Interactions between the ventilatory and chemosensory depressant effects of hypertoxia and DA administration were also studied. It is proposed that the reflex decrease in ventilation caused by DA injections provides a measure of the tonic chemosensory drive exerted upon the respiratory center.

Dissociation of hypoxia-induced chemosensory responses and catecholamine efflux in cat carotid body superfused in vitro.

1. To examine the correlation between chemosensory response and dopamine release induced by hypoxic stimulation, we studied carotid bodies excised from anaesthetized cats. 2. The carotid bodies with their carotid (sinus) nerves were superfused in vitro with modified Tyrode solution (pH 7.40, at 37.5 degrees C) equilibrated with 20 or 100% O2. The PO2 of the superfusing channel was monitored polarographically. The frequency of chemosensory discharges (fx) was recorded from the whole carotid nerve. Catecholamine (CA) efflux‐mostly consisting of dopamine‐was measured by high‐speed chronoamperometry, through Nafion‐coated carbon electrodes placed on the carotid body tissue. Chemosensory stimulation was induced by intrastream injections of NaCN, by superfusion with 100% N2‐equilibrated saline (lowering PO2 to 25‐40 Torr) or by flow interruption. 3. Low doses of NaCN increased fx, but had no measurable effect on CA efflux, while larger doses produced fast increases in fx, preceding delayed and prolonged increases in CA efflux. Repeated injections of NaCN, still increasing fx, gave reduced CA effluxes. 4. Switching to hypoxic superfusion for 6‐8 min produced large and fast fx increases, but delayed and prolonged augmentations of CA efflux. 5. Administration of three to four boluses of dopamine (7‐15 micrograms; augmenting CA concentration by up to 35 microM) initially decreased fx, after which hypoxic stimulation resulted in enhanced and faster CA effluxes, without changing the speed and intensity of chemosensory responses. 6. Flow interruptions induced fast increases in fx and delayed increases in CA efflux. Repeated flow interruptions produced similar increases in fx but progressively attenuated CA effluxes. 7. Our results suggest that CA efflux is not essential for hypoxia‐induced chemosensory excitation in the cat carotid body. They also suggest the presence of two pools of releasable CAs in the carotid body, one of slow turnover and release, and another of recently incorporated dopamine and fast release, both pools being rapidly depleted by repeated stimulation of the carotid body.