Mechanism of stimulation of aortic chemoreceptors by natural stimuli and chemical substances

Abstract

SUMMARY1. Impulses were recorded in single fibres of aortic chemoreceptors of cats anaesthetized with chloralose. There was no demonstrable difference between the responses of the endings of medullated and non-medullated fibres respectively to any of the natural stimuli, such as hypoxia, reduction in blood pressure, or reduction in 02 content. This indicates that the generator processes are qualitatively and quantitatively identical at the endings of both types of fibres.2. Most of the endings were practically silent while ventilating the lungs with air. The maximum frequency of discharge averaged over 4-10 sec while ventilating the lungs with 4 % 02 ranged from 1.5 to 24 impulses/sec; in most fibres (twenty-one out of twenty-six endings) it was less than 12 impulses/sec.3. All the chemoreceptors tested were considerably stimulated following administration of 0-2 or 2 % CO at a time when the 02 content was greater than 4 ml./100 ml.4. All the chemoreceptors were markedly and rapidly stimulated following circulatory arrest while the cat was ventilated with air. This stimulation fell considerably within 3 min of circulatory arrest. Very little or no excitation followed circulatory arrest while ventilating the cat with pure N2. These results suggest that excitation following circulatory arrest is not produced by a metabolite.5. There was a remarkable difference between the sensitivities of endings of medullated and non-medullated fibres to drugs. The former were either unaffected by relatively large doses of ACh (100-200 ,ug) or phenyl diguanide, or if they were stimulated, the excitation so produced was much less than that produced in endings of non-medullated fibres. This supports the hypothesis that drugs produce their effects by an action at the regenerative regions of the endings, i.e. regions where the nerve impulse is initiated (Paintal, 1964). It also indicates that ACh is not likely to be a transmitter in the normal processes of excitation of chemoreceptors.6. A mechanism of stimulation of chemoreceptors not involving metabolites is presented.

Communications

“…PO,, 4*1 kPa; Pco,, 7.3 kPa; pH 7-27 units) resulted in an increase in dP/dt max and, when the heart was not paced, an increase in heart rate; dP/dt max increased from 482 kPa sec-1 (mean; s. (Paintal, 1967) showed that, contrary to previous views (Bogue & Stella, 1935), excitation of carotid cheomoreceptors by ischaemia is only transitory. In the present experiments the time course of this effect was investigated and compared to that of anoxia and cyanide.…”

Interaction of Baroreceptor and Chemoreceptor Reflexes MODULATION OF THE CHEMORECEPTOR REFLEX BY CHANGES IN BARORECEPTOR ACTIVITY

“…We have examined data from a previous study (15 Catecholamines may sensitize chemoreceptors to other stimuli (8)(9)(10). We considered the possibility that release of catecholamines during hemorrhagic hypotension (7) The level of arterial pressure affects the rate of discharge of aortic chemoreceptors (10), although this effect is small (20). In the present studies, a direct effect of arterial pressure on aortic chemoreceptors may have contributed to potentiation of responses during hypotension and inhibition of responses during hypertension.…”

Mechanism of the ventilatory response to carbon monoxide.

“…This was confirmed by Haggard and Henderson in dogs (2), and more recently by Ayres et al in man (3), but Chiodi et al (4) found no ventilatory changes in human subjects with carboxyhemoglobin (COHb)1 levels as high as 50%. Similarly, there is no agreement as to whether or not carboxyhemoglobinemia increases neuronal activity from the peripheral chemoreceptors (5)(6)(7).…”