TOLERANCE TO SODIUM-NITROPRUSSIDE - STUDIES IN CULTURED PORCINE VASCULAR SMOOTH-MUSCLE CELLS

Background: Tolerance to nitrovasodilators, including sodium nitroprusside (SNP), is a clinical problem. SNP causes vasodilatation by releasing nitric oxide, which stimulates intracellular guanosine 3':5'-cyclic monophosphate (cGMP) accumulation in smooth muscle cells. This study examined tolerance to SNP in coronary smooth muscle cells by measuring intracellular cGMP formation. Methods. Smooth muscle cells were isolated from pig coronary vessels. Intracellular cGMP formation in response to SNP, nitroglycerine, and atrial natriuretic peptide (ANP) was quantitated by radioimmunoassay before and after chronic pretreatment. Results: The detection of tolerance of cells rechallenged with SNP required exposure to a minimum concentration of 1 mum for SNP for at least 60 min. In untreated cells, acutely applied 100 mum SNP increased the cGMP concentration to 300 +/- 8.6 pmol/mg protein (mean +/- SEM). The cGMP concentration in cells pretreated with 1 mum SNP for 60 min increased to 213 +/- 11 pmol/mg in response to 100 muM SNP (P < 0.01, compared to untreated cells). The pretreatment of cells for 60 min with increasing concentrations of SNP caused a sequential reduction in the acute concentration-response curves of cGMP to SNP. After a 6 h washout period, cGMP accumulation in response to a rechallenge with SNP returned to control levels. The acute concentration-response curve of vGMP to SNP in cells pretreated with 100 muM nitroglycerine for 60 min was reduced significantly. Pretreatment with 100 mum SNP for 60 min did not change the response of cAMP accumulation to isoproterenol or forskolin. However, the response of cGMP formation to ANP was potentiated after pretreatment. In addition, elevation of basal cAMP or cGMP concentrations, induced with isoproterenol or ANP, respectively, did not alter the response of cGMP formation to SNP. Conclusion: The study yielded the following findings: (1) reversible tolerance to SNP was induced in cultured vascular smooth muscle cells; (2) chronic exposure to nitroglycerine induced cross-tolerance to SNP; (3) cells that were tolerant to SNP showed a potentiated response to ANP.