Voltage-dependent calcium channels in young and old human red cells

Presence of subtypes of voltage-dependent Ca channels was investigated in young and old human red cells, employing immunological and flux-kinetics methods. Western blots showed specific reaction toward polyclonal rabbit antibodies raised against a highly conserved residue of alpha1 subunit of high-voltage activated Ca channels (pan alpha1) and against conserved residues of alpha1C and alpha1E subunits. No specific reaction was detected with antibodies against conserved residues of alpha1A, alpha1B, or alpha1D subunits. Only a single band (approx 260 kDa) was revealed on anti-pan alpha1 or anti-alpha1E blots, whereas two bands (200 and 230 kDa) were detected by anti-alpha1C exposure. Blots from old cells always showed diminished band intensity. Channel activity was assessed by studying the effect of voltage-dependent Ca channels blockers under conditions likely to alter the red cell membrane potential, through incubation in media of different composition. In a 150 mM NaCl + 5 mM KCl medium, blockers of L-, R-, and Q-type caused a 15-50% reduction of 45Ca influx into cells, which had the Ca pump inactivated by either exhaustive adenosine triphosphate depletion or presence of vanadate plus substrates. Additionally, some P/Qand N-type blockers also reduced Ca influx to various extents (25-60%). Old cells were generally insensitive to L-type but not to non-L-type blockers. Raising external K to about 70-80 mM reduced by 50-100% inhibition by L-type blockers. Incubation in a gluconate medium containing 150 mM Na+5 mM K practically abolished the action of L-type blockers, but only slightly reducing that by non-L-type. The results clearly demonstrate presence of L- and R-type Ca channels, apparently occurring in different functional states in young and old cells. Other non-L-type channels were also demonstrated only by pharmacological means. A possible physiological role for these channels is discussed.