Relation of membrane vesicles to volume control and Na+-transport in smooth muscle: studies on Na+-rich tissues.

Correlation between operation of a volume control system (independent of Na+-K+ -ATPase) and membrane vesicle structure was examined in strips of rat myometrium made Na-rich by incubation in Krebs-Ringer solutions at 5 degrees C. Some pieces of tissue were analyzed for Na+, K+, ATP and water contents before and after rewarming to 37 degrees C. Other pieces of tissue were prepared and examined in the electron microscope. The membrane vesicles along the surface of the muscle cells were reduced from the numberin fresh tissues made Na+-rich by overnight incubation in K+-free Ringer solution at 5 degrees C. Rewarming of Na+-rich tissues under conditions thought to inhibit Na+-K+-ATPase caused the loss of Na+ with water and with utake of K+ but the vesicles did not increase in number. Na+-K+ATPase activity was suggested to inhibit vesicle formation by competing for ATP. The loss of water and increase in vesicle number was Ca2+-dependent but did not require Na+. Inhibition of the volume control system by iodoacetamide, Ca2+-free solution and other procedures resulted in swelling of membrane vesicles. The membrane vesicles were suggested to be sites of a mechanochemical system of volume control and the mechanochemical step was suggested to involve discharge of membrane vesicles.