A comparison of ion concentrations, potentials and conductances of amphibian, bovine and cephalopod lenses

Abstract

The concentrations of Na+, K+ and Cl- in frog [Rana temporaria] and bovine lenses showed a normal intracellular ion distribution with the sum of the internal cations approximately equal to the external sum. In the cephalopod lens, the sum inside was much lower than that outside. The membrane potentials of from, Sepiola [atlantica] and bovine lenses were -63, -63 and -23 mV, respectively. A comparison of the electrical data with the Nernst potentials predicted from ion concentration data indicated that Na+, Cl- and K+ contributed to the membrane potential in frog and bovine. The membrane and Nernst potentials for K+ were equal in Sepiola. Substituting K+ for Na+ in the external medium depolarized lens potentials in all 3 spp. Estimates of the relative permeabilities of Na+, K+ and Cl- were obtained by fitting the Goldman-Hodgkin-Katz equation to the potential data. The K+ permeability was determined directly by 42K efflux measurements and values of 2.99, 9.83 and 3.13 (x-8 m .cntdot. s-1) were obtained for frog, Sepiola and bovine lenses, respectively. The effect of raising external K+ on the efflux rate constant was determined, and there was reasonable agreement between experiment and theory (Kimizuka-Koketsu) in frog and bovine lenses, but the Sepiola data indicated that the K+ permeability (PK) decreased by a factor of 2.6 when the external K+ was raised from 10 to 120 mM K+. The measured specific conductances, obtained using 2 internal microelectrodes, were 7.7, 15.9 and 9.9 (S[siemen]m-2) for frog, cephalopod and bovine lenses, respectively. These data compared with computed values (Kimizuka-Koketsu theory) of 7.5, 14.1 and 17.2 (Sm-2). The effect of increasing external K+ on the conductance was also tested, and there was good agreement between experiment and theory (assuming constant permeabilities) only in the amphibian lens. When the cephalopod data were corrected assuming a 2.6 fold decrease in PK for a 12-fold increase in K+ there was excellent agreement between experiment and theory. The bovine measured conductances were much lower than the theoretical values throughout the range of external K+ concentrations, and several explanations were proposed to account for the discrepancies.