Voltage clamp analysis of the ionic currents in uterine smooth muscle using the double sucrose gap method.

Voltage clamp experiments have been performed on uterine strips of pregnant rat using a double sucrose gap apparatus. The different components of the ionic membrane current have been analyzed. 1. After a transient fast capacitive current, and early inward current, which is time and voltage dependent, is observed. This current requires the presence of both sodium and calcium ions, and is reponsible for the rising phase of the action potential. Activation and inactivation parameters calculated on the basis of the Hodgkin-Huxley equations do not differ greatly from those described for nerve and muscle fibres. 2. A delayed outward current, which is time and voltage dependent, develops for depolarizations longer than 200 msec. This current, which presents and inactivation process for long lasting depolarizations, is not a pure potassium current, and other ions may participate in its development. Together with the inactivation of the initial conductance, it is responsible for the repolarizing phase of the action potential. It is possible to describe the activation in terms of the Hodgkin-Huxley equations. 3. It is proposed that this voltage clamp analysis of the ionic membrane currents of uterine smooth muscle justifies the use of this method for the testing of the effects of drugs and hormones.