Mechanism of Ca2+-dependent selective rapid K+-transport induced by propranolol in red cells

Summary Passive Ca²⁺ influx is gradually enhanced by 0.5 to 5mm propranolol in fresh and phosphate ester-depleted human red cells. In fresh cells the active Ca²⁺ efflux tends to counteract Ca²⁺ uptake. Membrane hyperpolarization, induced by the K⁺ transport that accompanies Ca²⁺ uptake, further enhances the rate of Ca² uptake. The dissociated, positively charged form of propranolol seems to be crucial in the increase of passive Ca²⁺ influx caused by the drug. The effect can be attributed to the release of structural Ca²⁺ from the membrane (lipids).The release of structural Ca²⁺ promotes the formation of the selectively K⁺-permeable membrane structure as well. The transitions of lipid structure responsible for the opening of the passive Ca²⁺ and K⁺ pathways, however, are not identical. The opening of the K⁺ pathways is prevented by certain highly lipid-soluble substances (chlorobutanol, heptanol, oligomycin, etc.), whereas the formation of the Ca²⁺ pathways is unaffected. Passive K⁺ transport is inhibited by high propranolol concentrations (more intensively at alkaline pH), whereas Ca²⁺ transport is promoted. A further difference between the passive K⁺ and Ca²⁺ pathways is that SH-proteins also seem to be involved in the formation of the K⁺ pathways, whereas they do not play a specific role in the opening of the passive Ca²⁺ channels. The additional Ca²⁺ binding that triggers the formation of the K⁺ pathways also seems to occur in the protein area of the inner membrane surface.