Abstract
The link between Ca2+ influx through the L-type calcium channels Cav1.2 or Cav1.3 and glucose- or KCl-induced [Ca2+]i mobilization in INS-1 cells was assessed using the calcium indicator indo-1. Cells responded to 18 mM glucose or 50 mM KCl stimulation with different patterns in [Ca2+]i increases, although both were inhibited by 10 μM nifedipine. Although KCl elicited a prolonged elevation in [Ca2+]i, glucose triggered oscillations in [Ca2+]i. Cav1.2/dihydropyridine-insensitive (DHPi) cells and Cav1.3/DHPi cells, and stable INS-1 cell lines expressing either DHP-insensitive Cav1.2 or Cav1.3 channels showed normal responses to glucose. However, in 10 μM nifedipine, only Cav1.3/DHPi cells maintained glucose-induced [Ca2+]i oscillation. In contrast, both cell lines exhibited DHP-resistant [Ca2+]i increases in response to KCl. The percentage of cells responding to glucose was not significantly decreased by nifedipine in Cav1.3/DHPi cells but was greatly reduced in Cav1.2/DHPi cells. In 10 μM nifedipine, KCl-elicited [Ca2+]i elevation was retained in both Cav1.2/DHPi and Cav1.3/DHPi cells. In INS-1 cells expressing the intracellular II-III loop of Cav1.3, glucose failed to elicit [Ca2+]i changes, whereas INS-1 cells expressing the Cav1.2 II-III loop responded to glucose with normal [Ca2+]i oscillation. INS-1 cells expressing Cav1.2/DHPi containing the II-III loop of Cav1.3 demonstrated a nifedipine-resistant slow increase in [Ca2+]i and nifedipine-resistant insulin secretion in response to glucose that was partially inhibited by diltiazem. Thus, whereas the II-III loop of Cav1.3 may be involved in coupling Ca2+ influx to insulin secretion, distinct structural domains are required to mediate the preferential coupling of Cav1.3 to glucose-induced [Ca2+]i oscillation.
- Received July 30, 2003.
- Accepted February 5, 2004.
- The American Society for Pharmacology and Experimental Therapeutics
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