Thermal modulation of cutaneous mucus discharge as a determinant of evaporative water loss in the frog, Rana catesbeiana

Summary

• 1. Cutaneous mucus glands of Rana catesbeiana discharge lumenal fluid onto the surface of the integument synchronously and periodically. Each discharge of fluid is a rapid and discrete event and can be viewed with magnification in the living animal. Discharge occurs in response to sympathetic nervous stimulation.

• 2. The frequency of mucus discharge depends upon central nervous impulses and increases over the approximate range of body temperature 20–28° C. Discharge frequencies during heating exceed steady state values at identical temperatures, and may be as high as 17/minute. Local heating of the thigh does not elicit changes in discharge frequency in that region, whereas local heating of the head does. Transections and lesions of the brain suggest that the anterior hypothalamus is involved in controlling mucus gland activity. Peripheral afferents appear to modify central impulses determining the frequency of mucus discharge.

• 3. Direct measurements of cutaneous evaporative water loss were made by recording humidity changes of air as it passed over an area of skin beneath a ventilated capsule. Frogs which frequently discharged mucus maintained steady states of evaporative water loss comparable to that of a free water surface. Frogs in which mucus gland activity was inhibited by sympathetic blockade demonstrated drying of the integument and declining rates of evaporative water loss.

• 4. It is suggested that thermal modulation of mucus discharge possibly functions to maintain a moist and viable integument during terrestrial basking.