L-type calcium channels blockers (CCB) suppress immune cell functions. CCB also influence the activity of V-type H+ pumps (V-ATPase). Plasma membrane V-ATPase is important for intracellular pH (pHi) regulation in macrophages (mφ). To explore the possibility that CCB can impact immune cells via pHi, we examined CCB effects on acid-base regulation in resident alveolar mφ.
Alveolar mφ were recovered from rabbits by bronchoalveolar lavage. The cells were treated with diltiazem, verapamil, or nifedipine. pHi and cytosolic calcium concentration [Ca2+]i were monitored spectrofluorometrically.
The blockers caused a small, but significant, decrease in baseline pHi and markedly inhibited pHi recovery following cytosolic acid-loading (NH4Cl prepulse). CCB effects on pHi recovery were mediated by V-ATPase inhibition. The effects were insensitive to amiloride and independent of external sodium or CO2/HCO3−. Channel blockade caused a 35% decrement in [Ca2+]i. However, it is unlikely that disruption of cytosolic calcium homeostasis was responsible for decreased V-ATPase activity. Treating the cells with calcium ionophores (A23187 or ionomycin) to increase the [Ca2+]i had no effect on V-ATPase activity. V-ATPase activity also was insensitive to inhibition of calcium-dependent protein kinase. Further, V-ATPase activity was unaffected by treating cells with cholicine to inhibit intracellular transport of vesicles or proteins. Calcium channel blockade had no effect on cellular ATP content, cell volume, or viability.CONCLUSIONS: CCB inhibited V-ATPase in resident alveolar mφ and impaired pHi regulation. The effects on V-ATPase do not appear to be linked to disruption of cytosolic calcium homeostasis. Rather, the findings may reflect direct actions of CCB to uncouple V-ATPase, as reported by Terland et al. (Eur J Pharmacol 207: 37, 1991).
The data suggest that V-ATPase inhibition and the consequent disruption of pHi regulation may be responsible, at least in part, for the documented immunosuppressive effects of CCB.
T.A. Heming, None.