Thiol-dependent passive K/Cl transport in sheep red cells: VII. Volume-independent freezing by iodoacetamide, and sulfhydryl group heterogeneity.

The sulfhydryl (SH) reagent iodoacetamide (IAAM) inhibits stimulation of Cl-dependent K transport in low K (LK) sheep red cells by another SH reagent, N-ethylmaleimide (NEM), without itself activating this transport pathway (J. Membrane Biol., 1983, 73:257-261). We now report that IAAM alone, acting with a kinetic slower than NEM, sharply reduced the capability of the Cl-dependent K transport system to regulate its activity in response to cell volume changes. This effect of IAAM did not depend on the cell volume maintained during chemical treatment, a fact ruling out that the reactivity of the SH groups with IAAM was a function of the volume-dependent turnover rate of the transporter. On the other hand, the prevention of the NEM-stimulatory effect on Cl-dependent K transport was found to be volume-dependent since 1) the rate with which IAAM blocked the subsequent NEM action was twice as fast in cells swollen in 250 mOSM as opposed to cells shrunken in 370 mOSM media, and 2) the dose response of the IAAM effect was different in swollen and shrunken cells. The action of IAAM with or without subsequent treatment with NEM seemed to be independent of cellular ATP which is required for full expression of the stimulatory modification of Cl-dependent K transport by NEM (Am. J. Physiol., 1983, 245:C445-C448). Clusters of SH groups on the Cl-dependent K transporter apparently react differently with IAAM and NEM when separately applied but, used in combination, reflect a complex volume-dependent effect that may reveal a "volume-sensing" component of the transport molecule.