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Potential And K+ Activity In Skinned Muscle Fibers. Evidence Against A Simple Donnan Equilibrium, R.E. Godt, C.M. Baumgarten

Physiology and Biophysics Publications

ABSTRACT

It has been suggested that potentials measured with conventional microelectrodes in chemically or mechanically skinned muscle fibers arise from a Donnan equilibrium due to myofilament fixed charges. This hypothesis was tested in mechanically skinned frog (Rana pipiens) semitendinosus fibers by measuring the distribution potential (Ed) between fiber and bath with 3 M KCI-filled microelectrodes and the K+ activity gradient (a'K/a) with K+ ion-selective microelectrodes (KISE). If skinned fibers are a Donnan system, Ed should become more positive as pH is decreased, altering the fixed charge on the myofilaments. Consistent with this expectation, Ed was -4.4, -0.6, and +4.8 mV …

Unidirectional Calcium And Nucleotide Fluxes In Sarcoplasmic Reticulum. I. Interpretation Of Flux Ratios For Different Reaction Schemes, J. J. Feher

Physiology and Biophysics Publications

The relation between unidirectional calcium and nucleotide fluxes was examined for different ATPase reaction schemes of the sarcoplasmic reticulum. The schemes considered differed in the order of sorption and desorption of calcium, ATP, and ADP. The results suggest that the theoretical relation between calcium and nucleotide fluxes depends on the reaction scheme and that experimental measurements can distinguish among them. The results obtained are generally valid and do not depend on assumptions of equilibrium or pseudoequilibrium between intermediate states of the pump.

Undirectional Calcium And Nucleotide Fluxes In Cardiac Sarcoplasmic Reticulum. Ii. Experimental Results, J. J. Feher, F. N. Briggs

Physiology and Biophysics Publications

Unidirectional calcium influx and efflux were evaluated in cardiac sarcoplasmic reticulum (SR) by 45Ca-"0Ca exchange at steady state calcium uptake in the absence of calcium precipitating anions. Calcium efflux was partitioned into a pump-mediated efflux and a parallel passive efflux by separately measuring passive efflux referable to the steady state. Unidirectional and net ATP-ADP fluxes were measured using [3H]-ATP -- ADP and [3H]- ADP -- ATP exchanges. Methods are presented that take into account changing specific activities and sizes of the nucleotide pools during the measurement of nucleotide fluxes. The contribution of competent and incompetent vesicles to the unidirectional and …