Module 12: Figure Li+ increases IP1 formation
Li+ enhances the accumulation of inositol monophosphate (IP1).
As a consequence of its uncompetitive action, Li+ exerts little inhibitory effect when the signalling pathway is operating within its normal operational range, but becomes increasingly effective as the signalling pathway is hyperstimulated. The graph on the left illustrates the accumulation of inositol monophosphate (IP1) in the blowfly salivary gland in response to 5-hydroxytryptamine (5-HT) in the presence or absence of 1 mM Li+. The physiological response of fluid secretion is included to show that the gland goes from a quiescent to a fully active state between 1×10−8 and 1×10−7 M 5-HT. In the absence of Li+, there is very little accumulation of IP1, even at very high 5-HT concentrations. In the presence of Li+, however, there is a large accumulation of IP1, which begins to rise exponentially when the 5-HT concentration increases above 1×10−7 M. What is revealing is that over the normal operational range of 5-HT concentration that regulates fluid secretion, there was almost no effect of Li+ on IP1 accumulation. It was only when the glands were hyperstimulated that Li+ began to inhibit the inositol monophosphatase, resulting in large accumulations of IP1.