VOLUME STATUS INFLUENCES ATRIAL PEPTIDE-INDUCED WATER CONDUCTIVITY CHANGES IN LEOPARD FROG MESENTERIC CAPILLARIES

1. Three sets of North American leopard frogs (Rana pipiens) were housed on damp sand (DS, n = 19), in fresh water (FW, n = 19), or with free access to both damp sand and fresh water (FA, n = 23) for a period of 5-6 days. At the end of this period pithed frogs were prepared for measurement of capillary hydraulic conductivity and samples of whole-blood, lymph and body cavity fluid were collected for measurement of haematocrit and protein content. 2. To measure hydraulic conductivity (L(p)), individual capillaries of the mesenteries of the three groups were cannulated and perfused with a minimum of two, and a maximum of three, frog Ringer solutions. The first solution, common to all experiments, contained dialysed bovine serum albumin (BSA) at a concentration of 10 mg ml-1. The second and third solutions also contained either 10 nM-atrial peptide (AP) or 1-mu-M-sodium nitroprusside (SNP). L(p) was determined from measurements of fluid filtration rate at a minimum of two different capillary pressures (Michel, Mason, Curry, Tooke & Hunter, 1974). 3. In vessels from damp sand-stored frogs (DS), the paired L(p) in the presence of 10 nM-atrial peptide (L(p,AP)) was 3.2 +/- 0.4-fold higher than L(p,BSA)) (n = 25). By contrast, in the FW group, L(p,AP)) did not differ significantly from L(p,BSA) (L(p,AP-FW)/L(p,BSA) = 1.3 +/- 0.2; n = 23). In frogs given free access to water and dry terrain the response to atrial peptide was similar to that observed in the DS group (L(p,AP-FA))/L(p,BSA) = 3.6 +/- 07; n = 23). Thus, storage in fresh water appears to have inhibited the acute L(p) response to atrial peptide observed in vessels from the FA or DS animals. 4. Atrial peptide and sodium nitroprusside elevate intracellular cyclic GMP levels by membrane receptor-dependent and -independent processes, respectively. No difference was observed in the L(p) response to 1-mu-M-perfusate sodium nitroprusside (L(p,SNP)) in DS (n = 7), FW (n = 8) or FA (n = 30) frogs (L(p,SNP)/L(p,BSA) ratios were 3.5 +/- 1.5-, 2.7 +/- 0.8- and 2.6 +/- 0.5-fold, respectively). Thus, the L(p) response to sodium nitroprusside was not affected by storage conditions. 5. The mean haematocrit of skin blood samples from the DS groups was 26.5 +/- 1.0% (+/- S.E.M.) compared to 19.1 +/- 1.2% from the FW groups. Plasma protein levels were 25.3 +/- 1.5 and 19.7 +/- 1.6 mg ml-1, respectively. Neither skin lymph nor body fluid protein levels differed between DS and FW groups. These data are consistent with a relative circulatory volume expansion of the FW frogs relative to the DS frogs. 6. Blood samples were taken from frogs housed with free access to both damp sand and fresh water (FA, n = 23). The mean haematocrit of these animals was 25.5 +/- 1.3%; plasma protein was 25.8 +/- 2.6 mg ml-1. These values were significantly greater than the FW frogs and did not differ from the DS frogs. 7. Overall, these results are consistent with specific desensitization of the exchange microvessels to the receptor-dependent atrial peptide hydraulic conductivity response following chronic plasma volume expansion.