SUMMARY1. 36C1-efflux rates were measured in depolarized fibre bundles (predominantly fast fibres) from m. semitendinosus of Rana temporaria in order to investigate the influence of chloride concentration (20-400 mM) and external pH (pH0, 5'5-1 16) on chloride self-exchange. Usually, the bundles were depolarized to a membrane potential of virtually zero ([Cl-]

SUMMARY1. The inhibition of 36C1 efflux with stilbene disulphonates, SD, has been studied under conditions of chloride equilibrium in depolarized fibre bundles from frog semitendinosi. The chosen probes were the aminoreactive derivative SITS and the derivative DNDS with no aminoreactive group. SD were added to the medium during 36C1 efflux allowing the estimation of fractional inhibition after a single 36C1 loading.2. Both probes inhibited chloride self-exchange reversibly within the pH range 5 5-9-5 under study.3. At SD concentrations above the half-inhibition concentration the inhibition reached a steady level with a time lag equal to that required for extracellular fluid change. The time constant for reversibility upon the removal of SD increased with decreasing pH, but rapid reversibility always appeared upon an increase of pH to 7-2. These findings suggest that SD may enter the membrane at low pH, but that the inhibitory action is confined to superficial membrane sites.4. The inhibitory power of both probes showed a pronounced pH dependence, pK 7. The half-inhibition concentration increased about 6-7 times when pH was lowered one unit from the pK value.5. The apparent affinity of SITS to the transport system was about 5 times higher than that of DNDS. The apparent dissociation constants at neutral pH were 8 5 x 10-5 M (SITS) and 4-5 x 10-4 M (DNDS). Both probes showed a maximal inhibition close to 100 % at neutral pH and -85 % at pH 5.5.6. The inhibition depended on the chloride concentration in a way consistent with competitive inhibition in both neutral and acid media.7. The results are consistent with the classical model of anion transport in frog muscle, suggesting that SD and chloride may compete for binding to a site with increasing anion affinity upon protonation; the results do not, however, exclude that the conductive and the non-conductive chloride transport modes in frog muscle are mediated by separate SD-sensitive transport pathways.J. M. SKYDSGAARD

0.: The time course of the latency relaxation as a function of the sarcomere length in frog and mammalian muscle. The dynamic properties of mammalian skeletal muscle. Gen Physioi 51:369-384. BARTELS, E. M . , JENSEN, P. & STEN-KNUDSEN. quire a time delay Of about 0*5-1.0 ms from the 0. 1976. The dependence of tension relaxation in lime the Ca'+-iOnS enter the filamentary lattice in skeletal muscle on the number ofsap-omeres in series. the overlap zone until the tension developing pro-Acta Physiol Scand 97: 476485.

SUMMARY1. The pH-and voltage-dependent Cl-conductance in frog muscle may be related to the chloride equilibrium potential, Vc1,eq, rather than to the absolute membrane potential, Vm. This hypothesis was tested in thin depolarized frog muscle fibre bundles by investigating the influence of pH on 36Cl-efflux during Cl-net efflux upon removal of external Cl-, which offers a state far from Cl-equilibrium by changing Vc1,eq instead of Vm.2. Upon the change from Cl-equilibrium at 20 mM-ClJ to Cl-net efflux at zero [Cl-]0, Vm changed only -10 mV, but the 36Cl-efflux rate increased about three times at pH 5-5 and decreased to less than one tenth of the equilibrium efflux rate at pH 9-8. The switch between 'acid' and 'alkaline' responses occurs at a pH between 6-4 ('acid response') and 7-2 ('alkaline response').3. Changing pH between 5-5 and 9'8 during Cl-net efflux showed an increase of 36Cl-net flux rate upon acidification and a decrease upon alkalination. The reactions are opposite to those seen by pH shifts at chloride equilibrium.4. The changes of net flux rate coefficients upon changes of pH were transient, especially the activation at low pH that relaxed significantly during about 10 min.5. The results are consistent with the notion that the state of the gcl mechanism in frog muscle is related to (VmVC1ejq) rather than to the absolute potential alone.

SUMMARY1. Inhibition of 36Cl-efflux in frog muscle by the stilbene disulphonates (SD) SITS (4-acetamido-4'-isothiocyanato-stilbene-2,2'-disulphonate) and DNDS (4,4'-dinitro-stilbene-2,2'-disulphonate) depends on the external pH, while the blocking of Clconductance with SITS is independent of pH. The 36Cl-efflux inhibition with DNDS has been studied in thin depolarized muscle fibre bundles in order to examine whether this difference is consistent with one transporter or reflects the existence of two mutually independent conductive and non-conductive SD-sensitive Clpathways.2. The 36Cl-efflux response to a sudden inward KCl gradient was studied. At high pH the efflux decreased as predicted for dominant conductive Cl-single-filing, and at low pH the efflux increased in agreement with dominant non-conductive Clantiport. DNDS inhibition resulted in the same response, an efflux decrease, at both high and low pH, suggesting a selective reduction of the non-conductive contribution at low pH.3. The inhibition of 36Cl-efflux as a function of the DNDS concentration at an external pH of 6-9 showed complex kinetics, which could be described as the sum of two Michaelis-Menten functions with different half-inhibition concentrations of DNDS.4. The results support the hypothesis that the stilbene disulphonate-sensitive Cltransport is mediated by two separate transporters rather than one.