Dual modulation of chloride conductance by nucleotides in pancreatic and parotid zymogen granules. |
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| Authors: | F Thévenod K W Gasser and U Hopfer |
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| Affiliation: | Department of Physiology and Biophysics, School of Medicine, Case Western Reserve University, Cleveland, OH 44106. |
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| Abstract: | The regulation of Cl- conductance by cytoplasmic nucleotides was investigated in pancreatic and parotid zymogen granules. Cl- conductance was assayed by measuring the rate of cation-ionophore-induced osmotic lysis of granules suspended in iso-osmotic salt solutions. Both inhibition and stimulation were observed, depending on the type and concentration of nucleotide. Under optimal conditions, the average inhibition measured in different preparations was 1.6-fold, whereas the average stimulation was 4.4-fold. ATP was inhibitory at 1-10 microM but stimulated Cl- conductance above 50 microM. Stimulation by ATP was more pronounced in granules with low endogenous Cl- conductance. The potency of nucleotides in terms of inhibition was ATP greater than adenosine 5'-gamma-thio]triphosphate (ATPS]) greater than UTP much greater than or equal to CTP much greater than or equal to GTP much greater than or equal to guanosine 5'-gamma-thio]triphosphate (GTPS]) much greater than or equal to ITP. The potency with respect to stimulation had the following order: adenosine 5'-beta gamma-methylene]triphosphate (AppCH2]p) greater than ATP greater than guanosine 5'-beta-thio]diphosphate (GDPS]). Adenosine 5'-beta gamma-imido]triphosphate (AppNH]p) was also stimulatory, and was more potent than ATP in the parotid granules, but less potent in the pancreatic granules. Aluminium fluoride stimulated Cl- conductance maximally at 15-30 microM-Al3+ and 10-15 mM-F. F was less effective at higher concentrations. Protein phosphorylation by kinases was apparently not involved, since the nucleotide effects (1) could be mimicked by non-hydrolysable analogues of ATP and GTP, (2) showed reversibility, and (3) were not abolished by the protein kinase inhibitors 1-(5-isoquinolinesulphonyl)-2-methylpiperazine (H-7) or staurosporine. The data suggest the presence of at least two binding sites for nucleotides, whereby occupancy of one induces inhibition and occupancy of the other induces stimulation. |
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