Characteristics of the passive 45Ca2+ transport by human blood platelets. Their dependence on the monovalent cation composition of the suspension media.

In the suspension of washed human platelets (32 degrees C, up to 30 min) we measured the uptake of 45Ca2+ in various media. We found that the substitution of Na+ by K+ or choline led to the stimulation of the uptake. According to the degree of the Na+ substitution we observed two phases of stimulation. The first one was observed in K(+)-containing media, but not in choline-containing media; the second one was observed in both K(+)- and choline-containing media. The transport in both phases was saturatable by Ca2+ and sensitive to other divalent cations. The saturating Ca2+ concentration and the sensitivity to other divalent cations were different in both phases. Dihydropyridine (nifedipine, nitrendipine) and phenylalkylamine (verapamil) calcium channel blockers were without effect in either medium up to 100 mumol/l. Indomethacin and acetylsalicylic acid were also without effect (100 mumol/l, and 1 mmol/l, respectively). On the other hand, veratridine (up to 75 mumol/l) stimulated the 45Ca2+ transport in Na(+)-rich medium but not in Na(+)-free medium. These results are in accordance with a notion that the first phase corresponds to a depolarization-stimulated Ca2+-transport, and the second one to the reversal of the Na/Ca exchange activity.