Loperamide: A positive modulator for store-operated calcium channels?

Abstract

The depletion of inositol trisphosphatesensitive intracellular pools of calcium causes activation of store-operated calcium (SOC) channels. Loperamide at 10-30 M has no effect on intracellular calcium levels alone, but augments calcium levels in cultured cells when SOC channels have been activated. In HL-60 leukemic cells, the apparent positive modulatory effect of loperamide on SOC channels occurs when these channels have been activated after ATP, thapsigargin, or ionomycin-elicited depletion of calcium from intracellular storage sites. Loperamide has no effect when levels of intracellular calcium are elevated through a mechanism not involving SOC channels by using sphingosine. Loperamide caused augmentation of intracellular calcium levels after activation of SOC channels in NIH 3T3 fibroblasts, astrocytoma 1321N cells, smooth muscle DDT-MF2 cells, RBL-2H3 mast cells, and pituitary GH 4 C 1 cells. Only in astrocytoma cells did loperamide cause an elevation in intracellular calcium in the absence of activation of SOC channels. The augmentation of intracellular calcium elicited by loperamide in cultured cells was dependent on extracellular calcium and was somewhat resistant to agents (SKF 96365, miconazole, clotrimazole, nitrendipine, and trif luoperazine) that in the absence of loperamide effectively blocked SOC channels. It appears that loperamide augments inf lux of calcium through activated SOC channels.The depletion of intracellular stores of calcium can result in the opening of calcium channels in the plasma membranes of cells (1). Such channels have been referred to as receptor-operated calcium channels, calcium-release-activated calcium channels, capacitative calcium entry channels, and store-operated calcium (SOC) channels. The mechanism(s) whereby depletion of inositol trisphosphate (IP 3 )-sensitive stores of calcium causes opening of SOC channels remains uncertain although several hypotheses have been advanced (2). SOC channels activate after receptor-mediated generation of IP 3 , which releases calcium from intracellular stores, or after treatment of cells with either the Ca 2ϩ -ATPase inhibitor thapsigargin, which blocks re-uptake of calcium into storage sites, or with the calcium ionophore ionomycin, which directly mobilizes calcium from storage sites. SOC channels can be blocked by imidazoles such as SKF 96365, clotrimazole, and miconazole and a small selection of other agents (3-6). Recently, loperamide was found to augment levels of intracellular calcium in HL-60 cells in which SOC channels were activated after P 2Y -receptor-mediated formation of IP 3 and release of intracellular calcium (6). The augmentation by loperamide of SOC channel-mediated elevation of intracellular calcium levels now has been shown to be a general phenomenon, occurring in several cell types after receptor-, thapsigargin-, or ionomycininduced activation of SOC channels. Loperamide appears to be a novel agent for the study of SOC channels and their functional role in cells. MATERIALS AND METHODSLoperamide, econazo...

Inhibitors of store-operated calcium channels: Imidazoles, phenothiazines, and other tricyclics

“…However, these imidazoles have also been reported to cause elevation of intracellular calcium and depletion of intracellular calcium stores [Benzaquen et al, 1995;Clementi and Meldolesi, 1996;Leung et al, 1996;Mason et al, 1993]. Inhibitors of voltage-dependent calcium channels, such as methoxyverapamil, nifedipine, diltiazem, cinnarizine, pimozide, proadifen, bepridil, and trifluoperidol at 30 µM had no apparent effect on SOC channels in HL60 cells [Daly et al, 1995;Harper et al, 1997].…”

“…Although various compounds are known to cause inhibition of SOC channels [Ciardo and Meldolesi, 1990;Clementi et al, 1992;Daly et al, 1995;Encabo et al, 1996;Guse et al, 1997;Harper et al, 1997] they are all relatively nonspecific, and 2-30 micromolar concentrations are required for significant inhibition. The major class of SOC channels inhibitors has been the imidazoles, including miconazole, clotrimazole, and SKF 96365.…”

Defects in inositol 1,4,5-trisphosphate receptor expression, Ca 2+ signaling, and insulin secretion in the anx 7(+/−) knockout mouse

“…In the absence of further interventions, the [Ca 2ϩ ] i would have stabilized in this region, presumably sustained by Ca 2ϩ entry through storeoperated calcium channels. However, on addition of 100 M loperamide (21), an activator of open but inactivated storeoperated calcium channels, the [Ca 2ϩ ] i rises and is sustained at the near maximal levels observed transiently after addition of carbachol. These data are representative of cells from 12 different normal littermate control animals studied at different times during a 12-month period.…”

“…Many ␤ cells occur as clusters, and such multiple signals occasionally occur in our hands. Finally, loperamide, an activator of store-operated Ca 2ϩ channels (21), is added, and virtually no response is noted. This experiment was repeated on 10 ␤ cells from different male and female anx7(ϩ͞Ϫ) animals, with essentially identical results in terms of suppressed responses.…”

Effects of Loperamide and Other Opioid-Related Substances on the Transcriptional Regulation of the Rat Pro-Opiomelanocortin Gene in AtT20 Cells

“…Interestingly, the potentiation was neither reproduced by W7 nor antagonized by nifedipine, suggesting that the positive and negative effects of loperamide are caused by different molecular mechanisms. Harper et al [22]have recently shown that loperamide has a VDCC-independent enhancing effect on calcium influx through the activation of a store-operated calcium channel. This may raise the possibility that loperamide enhances POMC gene expression in a calcium/calmodulin-independent manner, where it increases the calcium level from the intracellular calcium pool when the adenylate cyclase-cAMP signalling system is activated.…”