Aspects of rate-related hyperpolarization in feline Purkinje fibers.

Abstract

Rate-related hyperpolarization (RRH) in cat Purkinje fibers was studied by standard microelectrode techniques. Restrictions to ionic diffusion are less in this preparation than in dog preparations. The maximal hyperpolarization (VH) and time constant (.tau.) for the exponentially decaying hyperpolarization after cessation of rapid drive were used as aggregate indices for the underlying rate of Na pumping, the coupling ratio and membrane resistance. Changes in VH and .tau. in response to varying stimulation frequencies and durations of rapid drive, to cooling and addition of ouabain to changes in [K+]o (extracellular K+ concentration) and addition of Cs were used to assess the effects of RRH of Na loading, pump inhibition and short-circulating-K current, respectively. Steady state hyperpolarization was a linear function of stimulation frequency. Increased Na loading led to an increase in VH, but no change in .tau.. Onset and decay of RRH were symmetrical processes. The Q10 of .tau. and VH were 1.57 and 2.00, respectively. For a time after exposure to ouabain, .tau. and VH were unchanged; later, .tau. increased and VH decreased, but linearly with time. Relative membrane resistance decreased with rapid drive and increased with an exponentially decaying time course after cessation of rapid drive. Cs immediately increased VH, decrease .tau. and led to progressive deterioration of the fibers. Analysis of the steady state equation for RRH shows that a change in coupling ratio does not account for the steady state hyperpolarizations. Apparently the Q10 for VH is consistent with the Q10 for Na+, K+-ATPase.