Loperamide mobilizes intracellular Ca 2+ stores in insulin‐secreting HIT‐T15 cells

We have investigated the effects of loperamide on intracellular Ca 2+ stores and membrane K + channels in insulin‐secreting hamster insulinoma (HIT‐T15) cells. In cell‐attached patch‐clamp mode, loperamide (3–250 μ M ) activated large single‐channel currents. The loperamide‐activated currents were tentatively identified as Ca 2+ ‐activated K + channel (K Ca ) currents based on their single‐channel conductance (145 pS), apparent reversal potential, and insensitivity to tolbutamide. Smaller single‐channel currents with a conductance (32 pS) indicative of adenosine triphosphate‐sensitive K + channels (K ATP channels) were also recorded, but were insensitive to loperamide. Surprisingly, the loperamide‐activated currents persisted in the absence of extracellular Ca 2+ . Yet under these conditions, we still measured loperamide‐induced Ca 2+ increases. These effects are dose dependent. Loperamide had no effects in the inside‐out patch configuration, suggesting that loperamide does not directly activate the channels with large conductance, but does so secondarily to release of Ca 2+ from intracellular stores. Carbachol (100 μ M ), an agonist of muscarinic receptors, which mediates IP 3 ‐dependent intracellular Ca 2+ release, enhanced the effects of loperamide on K Ca channels. Both the putative K Ca currents and Ca 2+ signals induced by loperamide (with ‘0’ [Ca 2+ ] o ) were abolished when the intracellular Ca 2+ stores had been emptied by pretreating the cells with either carbachol or thapsigargin, an endoplasmic reticulum Ca 2+ ‐ATPase inhibitor that blocks reuptake of calcium. These data indicate that loperamide in insulin‐secreting β ‐cells evokes intracellular Ca 2+ release from IP 3 ‐gated stores and activates membrane currents that appear to be carried by K Ca , rather than K ATP channels.British Journal of Pharmacology (2003) 139 , 351–361. doi: 10.1038/sj.bjp.0705263