EFFECT OF PROTAMINE ON ION CONDUCTANCE OF ASCENDING THIN LIMB OF HENLE LOOP FROM HAMSTERS

To evaluate the contribution of paracellular shunt pathway in ascending thin limb (ATL) of hamsters, we examined the effect of protamine, a selective blocker of paracellular conductance, on salt-diffusion voltage (dV(T)) transmural resistance (R(T)) during in vitro microperfusion. Lumen-negative dV(T) generated on reduction of lumen NaCl concentration was increased further from -7.3 +/- 0.5 to -10.3 +/- 0.7 mV when 300-mu-g/ml protamine was added to the lumen, and calculated Na+/Cl- permeability ratio was decreased from 0.46 +/- 0.03 to 0.31 +/- 0.03. Although the effect of protamine persisted after removal of the agent from the lumen, addition of 30 U/ml heparin returned the dV(T) toward the control level. The effect of protamine was dose dependent from 30 to 300-mu-g/ml. Protamine also exerted its effect from the bath, and the effect was inhibited by heparin either from the lumen or from the bath. The inhibitory effect was almost the same when the orientation of imposed NaCl gradient was reversed. Inhibition of transcellular Cl- transport with 0.1 mM 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB) in the bath caused lumen-positive dV(T). This voltage was decreased significantly by protamine. Protamine markedly decreased the apparent transference number for Na+ but slightly increased the value for Cl-. Transmural cable analysis showed that 300-mu-g/ml protamine added to the lumen increased R(T) from 0.59 +/- 0.10 to 1.20 +/- 0.20 OMEGA.cm2, with the effect being reversed by 30 U/ml heparin. R(T) was significantly increased when 0.1 mM NPPB was added to the bathing fluid from 0.65 +/- 0.10 to 1.91 +/- 0.18 OMEGA.cm2. Although the baseline R(T) was extremely low, the same value was obtained when cable analysis was performed in low ambient NaCl solution. From these observations, we conclude that the paracellular shunt pathway in hamster ATL plays an important role in sodium-selective permeability. The extremely low R(T) is due to parallel existence of a high transcellular Cl- conductance and a high paracellular Na+ permeability.