Drugs Affecting Uterine Smooth Muscle

INTRODUCTION

Smooth muscle fibers in different organs are distinctly different from each other in their physical dimensions, organization into bundles or sheets, response to stimuli, characteristics of innovation, and function. Smooth muscle is responsible for the contractility of hollow organs, such as blood vessels, the gastrointestinal tract, the bladder and the uterus.

Specifically in the uterus, regulation of smooth muscle contraction is under the influence of an octapeptide known as oxytocin. Oxytocin is released through a mechanism called the "Ferguson reflex" during pregnancy (Rang et. al, 2003). These are impulses transmitted to the hypothalamus and neurohypohysis via afferent pathways from the pressure and rhythmic movement exerted by the foetus during labor (Gimpl & Fahrenholz, 2001). Oxytocin is also responsible for stimulating lactation and controlling post-partum hemorrhage. During the later stages of gestation, there is an increase in abundance of oxytocin receptors on uterine smooth muscle cells (Garofalo & Raymondo, 1995). The stimulation of the cervix and vagina induces the release of oxytocin, enhancing the contraction of uterine smooth muscle to facilitate parturition (Cree et al, 1999). Generally the uterus is insensitive to oxytocin however this dramatically increases with exposure to increased levels of oestrogen in the blood. The release of oxytocin consequently produces release of prostaglandins, which also helps to mediate uterine contraction. In cases where uterine contractions are not sufficient to complete delivery, physicians sometimes administer oxytocin to further stimulate uterine contractions (Leung, S.T. and Wathes, 1999).

Oxytocin is only one of a plethora of drugs that affects uterine smooth muscle. Vasopressin and acetylcholine (ACh) also have the ability to increase motility of the uterus, binding to V1 and V2 receptors (Bujis & Dantzer, 1990). Prostaglandin's like cloprostenol are used to maintain labor and to ripen the cervix and hence decrease smooth muscle contraction. In contrast to oxytocin and prostaglandin, stimulation of ОІ2-adrenoreceptors causes relaxation. Thus, drugs like adrenaline can be administered in an attempt to delay premature labor (Rang et. al, 2003).

The aim of this experiment was to study the responsiveness of the smooth muscle from a rat uterus to oxytocin and other drugs of