Na+/K(+)pump activity in photoreceptors of the blowfly Calliphora: A model analysis based on membrane potential measurements

Na+/K+-pump activity and intracellular Na+ and K+ concentration changes in blowfly photoreceptors are derived from intracellular potential measurements in vivo with a model based on the Goldman-Hodgkin-Katz theory for membrane currents. The relation between the intracellular Na+ concentration and the pump activity appears to follow a Hill function with a Hill coefficient of 1 and a maximal possible Na+ current due to pump activity of about -4 nA. The developed photoreceptor model incorporates the slow and fast voltage-dependent K+ channels of the blowfly photoreceptor of which the properties were taken from the literature.