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Background  The calcium-sensing receptor (CaSR) has been localized and characterized in numerous tissues throughout the body. In the mammalian gastrointestinal tract, the CaSR is known to act as a nutrient sensor and has recently been found to play a role in intestinal fluid and electrolyte balance. This study aims to demonstrate the functionality of the CaSR as a modulator of fluid secretion and absorption along the small intestine.    Methods  Small intestine regions (proximal, middle, and distal) were isolated from Sprague Dawley rats and loaded into an  ex vivo  intestinal perfusion device that provides independent intraluminal and extraluminal (serosa/basolateral) perfusion. The regions were perfused with 5 and 7 mM of Ca 2+ , both in the presence and absence of forskolin (FSK), a potent secretagogue. Control experiments were conducted with intraluminal perfusate containing standard Ringer-HEPES buffer with a physiological concentration of Ca 2+  (1 mM). A second set of comparison experiments was performed with intraluminal perfusates containing AC-265347, a CaSR activator and agonist, in the presence of FSK. In all experimental conditions, the intraluminal perfusate contained fluorescein isothiocyanate (FITC)-inulin, a nonabsorbable fluorescent marker of secretion and/or absorption. Intraluminal fluorescence signal was utilized as a measure of water movement at the start of the experiment and every 15 min for 90 min.    Results  Under physiological conditions, increasing the concentration of Ca 2+  in the luminal perfusate reduced intestinal fluid secretion in all regions. At a Ca 2+  concentration of 7 mM, net fluid absorption was observed in all regions. In the presence of FSK, 5 mM Ca 2+  significantly decreased fluid secretion and 7 mM Ca 2+  abolished FSK-induced fluid secretion. Intraluminal perfusion with 5 mM Ca 2+  was as effective as AC-265347, in reducing secretagogue-induced fluid hypersecretion in the proximal and middle regions.    Conclusion  This study concludes that apical CaSR is active along the small intestine. Its activation by Ca 2+  and/or calcimimetics reduces fluid secretion in a dose-dependent manner, with higher Ca 2+  concentrations, or application of a calcimimetic, leading to fluid absorption. We furthermore show that, in the presence of FSK, receptor activation abates FSK secretagogue-induced fluid secretion. This presents a new therapeutic target to address secretory diarrheal illnesses.

Activation of the Calcium Sensing Receptor Decreases Secretagogue-Induced Fluid Secretion in the Rat Small Intestine