Rubidium block and rubidium permeability of the inward rectifier of frog skeletal muscle fibres.

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Abstract
A 3-electrode voltage clamp method was used to investigate the Rb block of inward rectification in frog sartorius muscle fibers. In a solution containing 80 mM-K+, the K conductance increased with increasing hyperpolarization to 3.18 .+-. 0.11 mS [siemens] .cntdot. cm-2 (n = 17) when (V-VK) (total membrane potential-potassium membrane potential) was -150 mV. In the presence of Rb+, the conductance increased, fell and increased again with increasing hyperpolarization, i.e., the Rb block was first increased and then reduced by increasing hyperpolarization. Increasing [Rb]o [extracellular concentration] increased the block at all voltages. In a solution containing 80 mM-Rb+ (zero K+) inward currents were recorded when the membrane was hyperpolarized beyond .apprx. -60 mV. These currents, which were < 10% the amplitude of those in 80 mM-K solution, were blocked by tetraethylammonium ions. Experiments were carried out in solutions where [K]o and [K]i [intracellular concentration] were increased or where [K]o only was increased. The form of the relation between K conductance and membrane potential appeared to depend on [K]o. The magnitude of the conductance appeared to depend on [K]o and on [K]i. Concerning the block by Rb+, increasing [K]o appeared to enhance the release of Rb block under large hyperpolarizations. Increasing [K]o and [K]i reduced the Rb block at all membrane potentials. The results of experiments in the presence of Rb+ and Cs+ suggest that they do not compete with each other for a site at which to block inward rectification. Over a range of membrane potentials from -25 to -65 mV, the presence of Cs+ enhances the Rb block and vice versa. Single dissected muscle fibers (from semitendinosus) were used to measure sarcoplasmic resistivity in 80 mM-K solution and 160 mM-K (hyperosmotic) solution. The measured values were 163.2 .+-. 11.7 .OMEGA. .cntdot. cm and 136.1 .+-. 16.0 .OMEGA. .cntdot. cm, respectively (n = 7). A semi-empirical model is presented, supposing that Rb interacts with a site in the membrane to produce its blocking effect but is able to move into the sarcoplasm. Internal K+ is supposed to reduce the affinity of the site for Rb+; external K+ is able to enhance the movement of Rb+ into the sarcoplasm. The implications of these experiments for the nature of the permeability mechanism underlying inward rectification are discussed.