Calcium-induced transient potassium efflux in human red blood cells

The human red cell voltage-regulated cation channel. The interplay with the chloride conductance, the Ca(2+)-activated K(+) channel and the Ca(2+) pump

The activation/deactivation kinetics of the human erythrocyte voltage-dependent cation channel was characterized at the single-channel level using inside-out patches. It was found that the time dependence for voltage activation after steps to positive membrane potentials was slow ( t(1/2) about 30 s), whereas the deactivation was fast ( t(1/2) about 15 ms). Both activation and deactivation of this channel were also demonstrated in intact red cells in suspension. At very positive membrane potentials generated by suspension in extracellular low Cl(-) concentrations, the cation conductance switched on with a time constant of about 2 min. Deactivation of the cation channel was clearly demonstrated during transient activation of the Gárdos channel elicited by Ca(2+) influx via the cation channel and ensuing efflux via the Ca(2+) pump. Thus, the voltage-dependent cation channel, the Gárdos channel and the Ca(2+) pump constitute a coupled feedback-regulated system that may become operative under physiological conditions.

Erythrocyte Ca2+ handling in the spontaneously hypertensive rat, effect of vanadate ions

Cytosolic Ca2+ concentration ([Ca2+]i) and 45Ca2+ influx were investigated in erythrocytes from conscious spontaneously hypertensive rats (SHR) and their normotensive controls Wistar-Kyoto (WKY). [Ca2+]i was evaluated with fura-2 and intra- and extra-cellular calibration parameters were compared. Irrespective of the calibration parameters used, erythrocyte [Ca2+]i was always significantly higher in SHR than in WKY and Wistar rats (by 25 and 40%, p < 0.01 and 0.001). A rise of the external Ca2+ concentration from 1 to 2 mmol/l increased less [Ca2+]i in SHR than in WKY erythrocytes (17 vs 37%, p < 0.01). SHR erythrocytes incorporated more 45Ca2+ than those from WKY, with an initial rate of 45Ca2+ uptake higher by 57% than that of WKY erythrocytes (p < 0.05). Vanadate ions, after corrections of their quenching effect on red cell and fura-2 fluorescence signals, increased [Ca2+]i by 19% in WKY erythrocytes (p = 0.05), but did not modify the SHR values. They also increased 45Ca2+ accumulation and the initial rate of 45Ca2+ influx in WKY erythrocytes only (p < 0.01). This study indicates that, when compared to WKY rats, erythrocytes from SHR are characterized by higher [Ca2+]i values, higher initial rate of Ca2+ influx and low sensitivity to vanadate ions.

Effect of angiotensinogen concentration on renin activity following nephrectomy and adrenalectomy: implications in the activation of plasma prorenin

We have studied the effects of bilateral nephrectomy and adrenalectomy on angiotensinogen concentration, plasma renin activity, and total plasma renin activity obtained after trypsin activation in rats and compared with controls. Our results show that the plasma angiotensinogen concentration of nephrectomized (NX) rats is 5-fold higher than that of adrenalectomized (AX) rats. On the other hand, plasma angiotensinogen concentration of AX rats was about 2.5-fold lower than that of control rats. While NX rat plasma possess no renin activity, its mixing (1:1, v/v) with AX plasma results in 2-fold increase in renin activity over that observed for AX plasma alone. These results suggest that the apparent increase in renin activity upon mixing these two plasmas is at least partly due to an overall increase in angiotensinogen concentration in the mixture. To show that it is not due to activation of NX plasma prorenin by a convertase from AX plasma, prorenin-free NX rat plasma was prepared by using an anti-renin immunoaffinity column. When this prorenin-free NX plasma was mixed (1:1, v/v) with AX, again a 2-fold increase in renin activity was observed which is attributed to the overall increase in plasma angiotensinogen in the mixture. It is concluded that rat plasma prorenin is probably not activated within the circulation by a prorenin convertase from the rat kidney.

Does angiotensin regulate systemic conversion of prorenin to renin?

Within 12 hours of binephrectomy in rats, plasma prorenin rises about 250% above the pre-operative baseline and remains above-normal for at least 48 hours, indicating an extrarenal source of prorenin. Concurrently, active renin disappears, implying the loss of a renal "convertase" mechanism for prorenin activation. Such "convertase" activity was detected in incubates of renal cortical slices. To test the effect of angiotensin (Ang) on prorenin/"convertase" regulation, we infused Ang I (100 ng/kg/min) intraperitoneally for 24 hours and obtained evidence of "convertase" inhibition, as happened also following Ang II (2uM) addition to incubated cortical slices. Thus, the release and/or activity of "convertase" appears to be regulated by Ang in-vivo and in-vitro, suggesting that Ang controls not only direct renal renin release but also the secretion and/or activity of a renal "convertase" capable of producing additional renin from circulating prorenin pools.

Delayed activation of calcium pump during transient increases in cellular Ca2+ concentration and K+ conductance in hyperpolarizing human red cells

The net Ca2+ influx was increased in human red cells in suspension by adding moderate concentrations of the Ca2+ ionophore A23187, and due to the increased cellular Ca2+ concentration [( Ca]i) the K+ channels opened (the 'Gardos effect'). At low K+ concentration and with the protonophore CCCP in the buffer-free medium the cells hyperpolarized and the extracellular pH (pH0) increased, enhancing the A23187-mediated net Ca2+ influx. This elicited a prolonged response, viz. a primary transient increase of pH0 and [Ca]i followed by one or more spontaneous pH0 and [Ca]i transients. We explored the pump-mediated Ca2+ efflux by blocking the A23187-mediated Ca2+ flux with CoCl2 at appropriate times during the prolonged response. The Ca2+ pumping was higher during the descendent than during the ascendent phase of the primary transient at equal values of [Ca]i. The data were analyzed using a mathematical model that accounts for the prolonged oscillatory response, including pH0 and [Ca]i. In conclusion, the activation of the Ca2+ pump is delayed due to slow binding of cellular calmodulin, which is a hysteretic response to a rapid increase of the cellular Ca2+ concentration. This mechanism may be important for generation and execution of transient signals in other types of cell.