The messenger function of Ca2+: from PTH action to smooth muscle contraction

Endothelin and calciotropic hormones share regulatory pathways in multidrug resistance protein 2-mediated transport

The kidney of vertebrates plays a key role in excretion of endogenous waste products and xenobiotics. Active secretion in the proximal nephron is at the basis of this excretion, mediated by carrier proteins including multidrug resistance protein 2 (Mrp2). We previously showed that Mrp2 function is reduced by endothelin-1 (ET-1) through a basolateral B-type receptor, nitric oxide (NO), cGMP, and PKC (Notenboom S, Miller DS, Smits P, Russel FGM, Masereeuw R. Am J Physiol Renal Physiol 282: F458-F464, 2002; Notenboom S, Miller DS, Smits P, Russel FG, Masereeuw R. Am J Physiol Renal Physiol 287: F33-F38, 2004). This pathway was rapidly activated by several nephrotoxicants and appeared to be calcium dependent. In the present study, we studied the effect of the calciotropic hormones parathyroid hormone (PTH), PTH-related protein (PTHrP), and stanniocalcin (STC) to interfere with ET-regulated Mrp2 transport. Like ET-1, PTH reduces Mrp2-mediated transport by 40% in killifish renal proximal tubules. When given in combination, an additive effect was seen, which is partially reversed by the PKC inhibitor calphostin C. Recombinant PTHrP shows a comparable inhibitory effect, which is concentration dependent and additive to the inhibition by ET. STC fully reverses PTHrP-inhibited transport as does a guanylyl cyclase inhibitor. Finally, to confirm PTHrP bioactivity in a homologous assay, we performed immunolocalization and transport studies in sea bream kidney tubules. Mrp2 immunoreactivity was observed in approximately 40% of the tubules and is associated with the brush-border and apical plasma membrane of cells. Both proximal tubules and distal (collecting) tubules express the antigen. A highly significant 40% inhibition of Mrp2-mediated transport was observed with PTHrP in sea bream tubules. In conclusion, ET-regulated Mrp2 transport is influenced by calciotropic hormones and involves PKC and cGMP signaling.

Abnormal cell calcium homeostasis in type 2 diabetes mellitus: a new look on old disease

Cumulative evidence reveals that diabetes is a condition in which cell Ca2+ homeostasis is impaired. Defects in cell Ca2+ regulation were found in erythrocytes, cardiac muscle, platelets, skeletal muscle, kidney, aorta, adipocytes, liver, osteoblasts, arteries, lens, peripheral nerves, brain synaptosomes, retinal tissue, and pancreatic beta cells, confirming that this defect in cell Ca2+ metabolism is a basic pathology associated with the diabetic state. Though different defects in a variety of functions that regulate cell Ca2+ homeostasis were described in diabetes, the most common finding is an increase in [Ca2+]i levels. However, it is not clear whether the defect in cell Ca2+ metabolism in diabetes precedes or succeeds the overt diabetic condition. It is also not clear which of the multiple functions involved in cell Ca2+ regulation has the primary defect. Defects in cell Ca2+ metabolism may be significant for the observed pathologies in insulin secretion and insulin action in diabetes. They may also play an important role in the vascular complications seen in this condition, such as hypertension, atherosclerosis, and microangiopathy. Therefore, better understanding of the impairment in cell Ca2+ metabolism in diabetes may markedly enhance our understanding of this condition.