Enhanced Intracellular cGMP Concentrations and Lectin-Induced Lymphocyte Transformation

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Abstract
On the basis of recent reports that sodium azide, ascorbic acid, nitrosamines, and nitrosoguanidines elevated cGMP levels in some tissues, we examined their effects on cGMP metabolism in human lymphocytes. Sodium nitroprusside (NP), a nitrosoguanidine (MNNG), sodium azide, and ascorbic acid augmented intracellular cGMP concentrations many-fold in the presence or absence of calcium in the extracellular milieu. The response was usually maximal by 30 sec, persisted for at least 1 hr, and occurred in platelet-, granulocyte-, and monocyte-free preparations composed of greater than 90% T lymphocytes. In broken cell preparations these same agents augmented guanylate cyclase activity but had no effect on cAMP or cGMP phosphodiesterase. The magnitude of the increase in cyclase activity was in most cases proportional to the enhancement produced in intact cells, although sodium azide was a better stimulator of guanylate cyclase in broken cell preparations, and ascorbic acid in intact cells. With these agents as positive controls and four separate methods of cyclic nucleotide extraction before radioimmunoassay, significant and reproducible increases in cGMP were not demonstrable with Con A or E-PHA over a wide concentration range. The cGMP agonists did not in themselves produce lymphocyte mitogenesis or modulate E-PHA or Con A-induced transformation at optimal mitogenic concentrations. At supraoptimal concentrations of E-PHA or Con A (>25 µg/ml), NP increased thymidine incorporation and cell number. Cyclic GMP and its analogs alone neither enhanced thymidine incorporation nor modulated mitogen-induced lectin stimulation except for cGMP itself (1 and 0.1 mM) which increased 3H-thymidine transport but not cell number. These data do not support the concept of cGMP being a critical second messenger or having a permissive, positive regulatory role in lectin-induced transformation in human lymphocytes.

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