Circadian rhythm of extracellular pH in crayfish at different levels of oxygenation

Abstract

The extracellular pH regulation was studied in the crayfish Astacus leptodactylus (a night animal) as a function of circadian rhythm. The venus acid-base balance (ABB) was determined in the morning (10 a.m.–12a.m.) and in the evening (10 p.m.–12p.m.) at P O 2 ranging from 29 to 6 kPa and constant ABB in the water at 13°C. In the morning the venous pH (pH v ) was maintained constant by metabolic means independently of P O 2 from 29 to 10kPa. In the evening pH v again was constant and independent of P O 2 but it was more alkaline by 0.1 unit corresponding to a shift along the in vitro buffer line. At that time, the ventilation required for providing a unit quantity of O 2 ( i.e. the ventilatory requirement) increased more than for simply providing O 2. The related circadian changes of sensitivity of the ventilatory control system were assessed by comparing morning and evening ventilatory responses to 1-h periods of hypoxia and then hypercapnia. In the evening, the amplitude of the responses to both O 2 and CO 2 increased but the increase in CO 2 sensitivity was proportionally more important. This is consistent with the increase of ventilatory requirement and the related decrease of hemolymph P CO 2 during this period. It is concluded that in this animal there exists a circadian rhythm of extracellular pH that is achieved by controlling the CO 2 partial pressure in the hemolymph. Results are discussed in terms of O 2 transport processes and metabolic modulation through pH adjustments.