Cyclic AMP, calcium and control of cell growth

The role of cyclic AMP and calcium in the control of normal and tumour cell growth is considered in relation to the question whether cyclic AMP is a true mitogen or co-mitogen. It is proposed that cyclic AMP normally controls the cell cycle at a point in G1 phase only by virtue of its ability to exclude calcium required by cells to progress past this point into S phase. Therefore increased influx of calcium by other routes induced by various factors can bypass the inhibitory effect of cyclic AMP and stimulate growth. In these circumstances cyclic AMP or calcium may or may not facilitate further progress into S phase according to the metabolic requirements of individual cells. The relevance to cancer cells is considered.