Abstract
A small tonic contraction could be evoked by 3mM adenosine triphosphate (ATP) in the longitudinal muscle of pregnant rat myometrium which was incubated with the Ca-, Mg-free solution containing 2mM EGTA (40mM K). The amplitude of the tonic contraction was decreased by exposure to the solution within a few hours, but was again increased when 1-10mM Mg was added to the Ca-free solution. The ATP-induced contraction was increased for more than 7h during exposure to the Ca- free solution containing Mg ions. The ATP-induced contraction in the Ca- free (Mg-free) solution was also produced when the solution contained 1mM Mn or 1.5mM La. On the other hand, the tonic contraction evoked by 1mM Ca or Ba in the presence of 40mM K was depressed by ATP. The above results led us to propose that the ATP-induced contraction evoked in the Ca-free solution containing Mg was mediated by a process independent of Ca, and possibly mediated by a translocation of Mg in the cell. La and Mn could take the place of Mg in producing a tonic contraction caused by ATP application.
