The interplay of cutaneous water loss, gas exchange and blood flow in the toad, Bufo woodhousei: adaptations in a terrestrially adapted amphibian

Toads experiencing dehydrating conditions exhibit complex physiological and behavioral responses, some of which can potentially impact cutaneous gas exchange, an important component of total gas exchange. We measured the effect of dehydration on cutaneous gas exchange in the xeric-adapted toad Bufo woodhousei. First, two pharmacological agents were used to stimulate cutaneous blood flow - phentolamine (an alpha-blocker) and isoproterenol, a P-stimulant and powerful cardioaccelerator - to determine a relationship between cutaneous blood flow and water loss. Both drugs increased heart rate and blood pressure, and caused visually evident extensive vasodilation of the skin. Untreated toads in a dry air stream took an average of 10.1 +/- 0.7 h to dehydrate to 80% body mass, while animals treated with isoproterenol and phentolamine requires only 7.2 +/- 0.8 h and 7.4 +/- 0.9 h, respectively. Rehydration, which was more rapid than dehydration, was similarly accelerated in pharmacologically treated toads. Cutaneous gas exchange (M-O2, M-CO2) and (CO)-O-18 diffusing capacity ((DSkinCO)-O-18) were then examined in unanesthetized toads under different states of body hydration. Blood gases and hematocrit were measured separately but under identical conditions. In fully hydrated toads at 23-25 degreesC, cutaneous gas exchange values were: M-O2 = 1.43 +/- 0.47 mumol g(-1) h(-1), M-CO2 = 1.75 +/- 0.85 mumol g(-1) h(-1), and the respiratory exchange ratio R = 1.36 +/- 0.56 (N=6, mean + 1S.D.). (DSkinCO)-O-18 was 0.48 +/- 0.03 mumol g body mass(-1) h(-1) kPa. Following an enforced 20-25% loss of body water, (DSkinCO)-O-18 fell by nearly 50% to 0.28 +/- 0.09 mumol g(-1) h(-1) kPa. However, cutaneous M-O2, M-CO2 and R were unchanged at 1.48 +/- 0.15 mumol g(-1) h(-1), 1.72 +/- 0.29 mumol g(-1) h(-1) and 1.13 +/- 0.08 mumol g(-1) h(-1), respectively. Partial pressure of arterial (sciatic) oxygen, Pa-O2, normally about 12-13 kPa, remained unchanged by dehydration, but Pa-CO2 increased about 250% from 0.93 +/- 0.27 up to 2.27 +/- 0.93 kPa. The fall in (DSkinCO)-O-18 during dehydration presumably results at least in part from decreased cutaneous blood flow, possibly in an attempt to reduce the transcutaneous water loss that would otherwise result during dehydrating conditions. Concurrently, cutaneous M-CO2 is maintained under dehyrdating conditions by a greatly increased Pa-CO2 diffusion gradient across the skin. Thus, Bufo woodhousei appears able to restrict cutaneous blood flow without compromising vital cutaneous CO2 loss.