Regulation of Ca2+ channel and phosphatase activities by polyamines in intestinal and vascular smooth muscle – implications for cellular growth and contractility

Polyamines added extracellularly to intestinal and vascular smooth muscle cells cause relaxation through inhibition of Ca²⁺ channel activity. Intracellularly applied polyamines also affect Ca²⁺ channel properties. Polyamines do not readily pass over the plasma membrane because of their positive charges but in permeabilized smooth muscle preparations they have free access to the cytoplasm. In this system they increase sensitivity of the contractile machinery to Ca²⁺ through inhibition of myosin phosphatase activity. The magnitude of Ca²⁺ channel and phosphatase inhibition depends on the number of positive charges on the polyamine molecule. Polyamines have an obligatory, but yet undefined, role in regulation of cell growth and proliferation. Several groups of protein kinases, such as tyrosine and mitogen activated protein (MAP)‐kinases transmit the growth signal from the plasma membrane to the cell nucleus where mitosis and protein synthesis are initiated. The data reviewed here show that polyamines may affect such signal transmission via inhibition of phosphatase activity.