Cholecystokinin and gastrin induce cell contraction in pig ileum by interacting with different receptor subtypes

The aim of this study was to determine the cholecystokinin (CCK) receptor subtype involved in the direct myogenic effect of CCK on pig ileum. Smooth muscle cells were dispersed from pig ileum circular muscle layer and incubated in the presence of various concentrations of CCK agonists and antagonists. Contraction was assessed by measuring the length of 50 cells and expressed as the percentage decrease in cell length from control. Maximal contraction varied between 19% +/- 3% (gastrin II, 10 nmol/L) and 26% +/- 3% [CCK octapeptide (CCK-8), 10 nmol/L]. EC50 for CCK tetrapeptide (CCK-4) was the same than for pentagastrin (30 pmol/L), which were more potent than CCK-8 (100 pmol/L) and unsulfated gastrin 17 (100 pmol/L), which in turn were more potent than unsulfated CCK heptapeptide (CCK-7; 300 pmol/L) and sulfated gastrin II (300 pmol/L). The maximal contraction induced by synthetic analogs of CCK was 22% +/- 1% for 1 nmol/L JMV 170 and 23% +/- 1% for 10 nmol/L JMV 180. EC50 was 10 pmol/L for JMV 170 and 800 pmol/L for JMV 180. Contraction induced by 10 nmol/L CCK was inhibited as follows: L 365,260 half maximal inhibition (IC50) = 1 nmol/L greater than L 364,718 (IC50 = 90 nmol/L) greater than proglumide (IC50 = 1 mumol/L). Contraction induced by 10 nmol/L unsulfated gastrin 17 was inhibited as follows: L 365,260 (IC50 = 1 pmol/L) greater than L 364,718 (IC50 = 60 nmol/L) greater than proglumide (IC50 = 4 mumol/L). Removal of Ca2+ from the extracellular medium did not alter the contraction induced by CCK-8 (10 nmol/L) but impaired the contraction induced by unsulfated gastrin 17 (10 nmol/L) -56% in Ca(2+)-free medium, -77% in Ca(2+)-free medium plus 2 mmol/L EGTA, and -70% in the presence of 1 mumol/L nifedipine. These results show that the CCK receptor of pig ileum smooth muscle cells is closely similar to the B receptor and is not dependent on an influx of extracellular Ca2+ to induce cell contraction. By contrast, gastrin could act through a specific receptor subtype, the "gastrin receptor," triggering a Ca2+ influx into the cell to induce cell contraction.