Leon Hurwitz

A Calcium Pump in Vascular Smooth Muscle

A microsomal cell fraction derived from the intimal-medial layer of rabbit aorta takes up calcium in the presence of magnesium and adenosine triphosphate. The rate of uptake of calcium is slower than that observed in skeletal muscle microsomes. Uptake of calcium by mitochondria from the aorta is even more limited and, unlike microsomal uptake, is inhibited by azide.

Localization of Calcium Pump Activity in Smooth Muscle

A microsomal fraction isolated from longitudinal smooth muscle of guinea pig ileum actively sequesters calcium ion in the presence of magnesium and adenosine triphosphate in a fashion previously described for microsomes of the rabbit aorta. This activity in guinea pig ileum appears to be associated primarily with the plasma membrane as is found in the red cell. By contrast the uptake of calcium in aortic smooth muscle appears to be associated to an appreciable extent with intracellular membranes, possibly analogous to the sarcoplasmic reticulum of skeletal muscle.

Energy-dependent calcium uptake activity of microsomes from the aorta of normal and hypertensive rats

Energy-dependent calcium uptake activity of microsomes isolated from the rat aorta has been characterized. The microsomes consist of smooth membrane vesicles which in the presence of MG-ATP as an energy source continuously sequester calcium over a 60-min period. This calcium uptake is greatly stimulated by oxalate anion which serves as a calcium trapping agent. Unlike the calcium uptake of mitochondria this uptake is not inhibited by sodium azide. Sucrose density gradient analysis of the microsomal calcium uptake suggests that the system is associated with the sarcoplasmic reticulum. In presence of 5 mM Mg-ATP and 20 muM calcium approximately 38 nmol of calcium per mg of microsomal protein are taken up in 20 min. In the absence of ATP, less than 2 nmol of calcium per mg of protein are taken up in the first 2 min with no further uptake of calcium in subsequent time periods. When calcium uptake activity is plotted against calcium or ATP concentration of the medium, half maximal activity is calculated for 24.3 muM calcium and for 1.6 mM ATP. The calcium uptake characteristics of the rat aorta microsomes are compatible with a postulated role in the relaxation of the vascular smooth muscle and the provision of an intracellular calcium store for muscle contraction. Aorta microsomes from SHR rats (a genetic strain that is spontaneously hypertensive) have a significantly reduced uptake when compared with the corresponding nonhypertensive control strain. The level of calcium and ATP for half maximal activity of the rat aorta microsomal calcium uptake system is approximately the same in the SHR and the control strain. The rate of release of calcium from rat aorta microsomes is apparently identical in SHR strain and control. The calcium uptake activity of kidney and liver microsomes isolated from the SHR strain and control. The calcium uptake activity of kidney and liver microsomes isolated from the SHR rat appears to be identical to that found in the control strain.

Mechanical Responses of Intestinal Smooth Muscle in a Calcium-Free Medium.

Summary Longitudinal fibers from the guinea pig ileum undergo a transient increase in muscle tone when they are transferred from a physiological salt solution to a calcium-free solution, The magnitude and duration of the contractile response is enhanced if the muscle is preincubated in a high calcium medium (36 mM). The length of time that the muscle is preincubated in the high calcium medium influences the magnitude of the response as well as the amount of intracellular calcium accumulated by the fibers. The mechanical changes initiated by removing extracellular calcium ions are accompanied by a pronounced increase in the unidirectional efflux of calcium ions. The addition of acetylcholine or a high potassium medium enhances the tone of a muscle that is partially contracted in a calcium-free medium. The addition of 1.8 mM CaCl2 depresses it. However, when the same concentration of CaCl2 is added in the presence of acetylcholine or a high potassium medium it enhances muscle tone even more. We inferred from these data that acetylcholine, a high potassium medium, and a calcium-free medium, by increasing membrane permeability, accelerate the migration of calcium ions from an intracellular depot to the cytoplasm. The results suggest that they also accelerate the movement of calcium ions between the external solution and the cytoplasm of the muscle fiber.