Membrane properties of adult mammalian cardiac muscle are difficult to define mainly because of experimental complications arising from complex packing of myocytes in the tissue matrix. Isolated feline myocytes were used in the present study to avoid these complications. The objectives of this study were to define the functional relationship between passive unidirectional transmembrane potassium (K+) fluxes, membrane permeability to K+ (PK), and membrane K+ (Ko) dependency of this relationship. Passive (ouabain-insensitive) components of unidirectional K+ fluxes were measured with 42K, and membrane potential (Em) and membrane (slope) conductance (gm) were measured with electrophysiological techniques. Myocytes studied in solutions with 5 mM K+o had normal resting potentials (-81 +/- 1 mV). The input resistance and membrane time constant were 2.72 +/- 0.47 X 10(-7) omega and 7.01 +/- 1.0 ms, respectively. When K+o was lowered Em hyperpolarized, input resistance (Ri) increased, and K+ fluxes decreased. When K+o was increased Em depolarized, Ri decreased, and K+ fluxes increased. These data were combined to determine whether K+ fluxes obey the independence principle and to calculate PK and gK. The results obtained support the idea that 1) unidirectional K+ fluxes do not obey the independence principle, 2) PK is much greater than the membrane permeability to other ions, and 3) the gK calculated from passive K+ fluxes was similar to the gm measured electrically (at all K+o's tested).
Houser et al. (Wed,) studied this question.