Excitation of skinned muscle fibers by imposed ion gradients. III. Distribution of permeant ions in unstimulated and stimulated fibers

Ion gradients imposed across an internal membrane system stimulate skinned muscle fibers; to evaluate the sarcoplasmic reticulum (SR) as the primary target site, SR polarization under resting and stimulatory conditions was assessed from fiber uptake of permeant probe ions. Solvent spaces were estimated from simultaneous [14C]urea (U) or [3H]deoxyglucose (DOG) uptake in segments of fibers from bullfrog semitendinosus muscle, skinned by microdissection. The distribution spaces, i.e., virtual solvent volumes at bath concentrations (Vu and VDOG), of these uncharged probes correlated well with the protein content of the same segments, which validated the tracer methodology for volume normalization. The membrane-bounded volume fraction (Vm), derived from the difference between total solvent volume (Vs) and the non-membrane-bounded solvent volume (Vc), was sufficient to detect appreciable SR ion accumulation. The Vm estimated from the difference between VU and VDOG assayed simultaneously with 2 or 5-6 min exposures was 10-11%, which is consistent with the morphometric volume fraction (mostly SR) in frog fibers; however, the change in this difference after membrane permeabilization corresponded to Vm only 5%. The change in permeant ion distribution space caused by member permeabilization was used to assess SR membrane polarization, assuming the free ions distribute across the intact membrane according to the Nernst ratio. Resting polarization (SR lumen positive) was assessed from [14C]SCN- or [14C]propionate- distribution spaces in unstimulated fibers, expressed relative to VDOG (assayed simultaneously). The ratios for (a) [14C]SCN- space (carrier 2 mM) and (b) [14C]propionate- space (carrier 120 mM) were not decreased by membrane permeabilization. This indicated that anion distribution was independent of membrane integrity and did not reflect an SR transmembrane potential, although a was more and b was less than 1. Polarization under stimulatory conditions (lumen negative) was assessed from 86Rb+ distribution, before and after an imposed ion gradient (choline Cl replacement of K methanesulfonate (KMes) at constant [K+] [Cl-]) that theoretically could generate a 48-fold transmembrane cation ratio; Ca release was minimized by EGTA. The ratio of 86Rb+ space to VU, greater than 1 in KMes (120 mM K, the effective carrier), was higher in choline Cl (2.5 mM K) but not decreased by membrane permeabilization; this indicated that 86Rb+ distribution did not reflect an SR transmembrane potential. Similar results in the presence of valinomycin ruled out the possibility of inadequate 86Rb+ equilibration.(ABSTRACT TRUNCATED AT 400 WORDS)