| Abstrakt: |
Phospholipase C (PLC) activity was investigated by stimulation of membrane preparations obtained from insulin (beta-TC3)-, somatostatin (Rin 1027-B2)-, and glucagon (INR1-G9)-producing pancreatic cell lines using the non-hydrolyzable GTP analogue GTPgammaS alone, the C-terminal octapeptide cholecystokinin (CCK-8), or gastrin. All compounds caused a significant 2- to 4.4-fold stimulation of PLC activity in the different cell lines, which was diminished by the non-hydrolyzable GDP analogue GDPbetaS. CCK receptor subtypes were characterized by radioligand binding experiments. High-affinity binding sites for tritiated CCK(A) receptor antagonist L-364,718 (K(d) = 0.24 nM) and tritiated CCK(B) receptor antagonist L-365,260 (K(d) = 0.13 nM) were only present in Rin 1027-B2 cells. High-affinity binding sites for both ligands were not found in beta-TC3 or INR1-G9 cells. Competition binding experiments with non-labeled CCK receptor antagonists CR 1505 (CCK(A) receptor-selective) and CR 2945 (CCK(B) receptor-selective), as well as microphysiometry experiments, resulted in the same receptor distribution. Reverse transcriptase-polymerase chain reaction confirmed the CCK receptor distribution pattern for Rin 1027-B2 cells, but in addition showed the existence of CCK(B) receptors in beta-TC3 cells. Immunoblocking experiments with C-terminal antibodies against different G-protein alpha-subunits demonstrated inhibition of CCK-stimulated PLC activity in beta-TC3 cells by G(q/11)alpha antiserum (70%), in Rin 1027-B2 cells by G(q/11)alpha antiserum (70%) and G(i)-3alpha antiserum (23%), and in INR1-G9 cells by G(q/11)alpha antiserum (60%) and G(o)alpha antiserum (45%). We conclude that CCK receptor subtypes with different G-protein-coupling specificities to PLC are present in the different hormone-secreting cells of the endocrine pancreas. |
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