Cell alkalosis elevates cytosolic Ca2+ in rabbit resident alveolar macrophages

Disruption of cellular acid-base status alters the host defence functions of alveolar macrophages (m(phi)). These pH effects might be mediated by pH-sensitive changes in the signalling pathways of the effector functions of m(phi). The present study examined the effects of intracellular pH (pH(i)) on the free cytosolic calcium concentration ([Ca2+](i)), an important second messenger for cell functions. [Ca2+](i) and pH(i) of rabbit resident alveolar m(phi) were measured using fluorescent dyes. With extracellular pH (pH(o)) of 7.4, the steady-state pH(i) and [Ca2+](i) were approx. 7.14 and 123 nM respectively. Incubation at pH(o) 6.8 caused a sustained cytosolic acidosis, but did not affect [Ca2+](i). Likewise, [Ca2+](i) was unchanged when m(phi) at pH(o) 7.4 were acidified using bafilomycin A(1) or sodium propionate. In contrast, [Ca2+], was markedly sensitive to cytosolic alkalosis. Exposure to NH4Cl at pH(o) 7.4 caused transient increases in both pH(i) and [Ca2+](i). The Ca2+ response was mediated by release of intracellular Ca2+ from thapsigargin-sensitive stores and was potentiated by capacitative entry of extracellular Ca2+. Incubation at high pH(o) values (>7.4) produced sustained increases in pH(i) and [Ca2+](i). The sustained elevation of [Ca2+](i) was consistent with pH-sensitive inhibition of plasma-membrane Ca2+-ATPase. The response to high pH(o) was unaffected by blockade of L-type or receptor-operated Ca2+ channels with nifedipine or SKF-96365, and was independent of extracellular Na+. The findings indicate that pH impacts cytosolic Ca2+ homoeostasis at multiple levels. The data suggest that cellular acid-base status can influence Ca2+-dependent signalling events in resident alveolar m(phi), especially during alkaline disruptions of pH(i).