Chronic hypocalcemia of vitamin D deficiency leads to lower intracellular calcium concentrations in rat hepatocytes

Abstract

Several lines of evidence indicate that calcium deficiency is associated with cellular defects in many tissues and organs. Owing to the large in vivo gradient between ionized extra- and intracellular Ca2+ concentrations ([Ca2+](i)), it is generally recognized that the prevailing circulating Ca2+ does not significantly affect resting cytosolic Ca2+. To probe the consequences of hypocalcemia on [Ca2+](i), a model of chronic hypocalcemia secondary to vitamin D (D) deficiency was used. Hepatocytes were isolated from livers of hypocalcemic D-deficient, of normocalcemic D3-repleted, or of normal control rats presenting serum Ca2+ of 0.78±0.02, 1.24±0.03, or 1.25±0.01 mM, respectively (P < 0.0001). [Ca2+](i) was measured in cell couplets using the fluorescent probe Fura-2. Hepatocytes of normocalcemic D3-repleted and of normal controls exhibited similar [Ca2+](i) of 227±10 and 242±9 nM, respectively (NS), whereas those of hypocalcemic rats had significantly lower resting [Ca2+](i) (172±10 nM; P < 0.0003). Stimulation of hepatocytes with the α1-adrenoreceptor agonist phenylephrine ilicited increases in cytosolic Ca2+ leading to similar [Ca2+](i) and phosphorylase a (a Ca2+-dependent enzyme) activity in all groups but in contrast to normocalcemia, low extracellular Ca2+ was often accompanied by a rapid decay in the sustained phase of the [Ca2+](i) response. When stimulated with the powerful hepatic mitogen epidermal growth factor (EGF), hepatocytes isolated from hypocalcemic rat livers responded with a blunted maximal [Ca2+](i) of 237.6±18.7 compared with 605.2±89.9 nM (P < 0.0001) for their normal counterparts, while the EGF-mediated DNA synthesis response was reduced by 50% by the hypocalcemic condition (P < 0.03). Further studies on the possible mechanisms involved in the perturbed [Ca2+](i) homeostasis associated with chronic hypocalcemia revealed the presence of an unchanged plasma membrane Ca2+ ATPase but of a significant decrease in agonist- stimulated Ca2+ entry as indicated using Mn2+ as surrogate ion (P < 0.03). Our data, thus indicate that, in rat hepatocytes, the in vivo calcium status significantly affects resting [Ca2+](i), and from this we raise the hypothesis that this lower than normal [Ca2+](i) may be linked, in calcium disorders, to inappropriate cell responses mediated through the calcium signaling pathway as illustrated by the response to phenylephrine and EGF.