In amphibians, salt and water balance is regulated, in part, by the cutaneous uptake of Na+ and water from the environment. The rate of cutaneous salt and water uptake may be related to cutaneous blood flow, but it is not known if cutaneous blood flow is regulated for salt and water balance. To assess that possibility, we determined in Bufo woodhousei the effects on the cutaneous circulation of (1) dehydration, (2) water contacting the skin, and (3) different NaCl solutions contacting the skin. The ventral pelvic skin was observed with intravital fluorescent video microscopy (n = 8). Measurements were made on toads in three hydration states: hydrated, moderate dehydration (water loss = 15.8% ± 1.7% of original body weight), and severe dehydration (water loss = 25.4% ± 1.0% of body weight). Fluxes and velocities of ethidium-labeled red cells in the capillaries were determined (1) with the skin surface dry, (2) 4 min after the skin was superfused with water, and (3) 4 min after superfusion with 0%, 0.7%, 2.0%, and 5% NaCl solutions. Capillary red cell flux (P < 0.001) and velocity (P = 0.02) varied inversely with dehydration: red cell fluxes were 93 ± 14, 41 ± 10, and 13 ± 5 cells · s⁻¹, and cell velocities were 0.78 ± 0.15, 0.50 ± 0.09, and 0.18 ± 0.10 mm · s⁻¹ in dry skin of hydrated, moderately dehydrated, and severely dehydrated toads, respectively. Superfusing the skin with water had no effect on the skin circulation in hydrated toads (P > 0.15) but increased cell fluxes (P < 0.05) to 56 ± 8 and 34 ± 4 cells · s⁻¹ and cell velocities (P < 0.02) to 0.65 ± 0.09 and 0.34 ± 0.10 mm · s⁻¹ in moderate and severe dehydration, respectively. Superfusate [NaCl] up to 2% had no effect (P > 0.1), but 5% NaCl reduced both cell fluxes (P < 0.05) and velocities (P < 0.05) in all three hydration states. The results indicate that (1) dehydration reduces perfusion in ventral pelvic skin, (2) water contacting the ventral pelvic skin increases perfusion only during dehydration, and (3) only concentrations of NaCl probably not normally encountered affect ventral pelvic skin perfusion. These responses should facilitate water uptake in dehydrated animals and reduce water loss and salt gain should animals come in contact with a substrate > 5% NaCl.
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