The anococcygeus muscle is a paired smooth muscle, arranged in parallel bundles to form a sheet allowing easy diffusion of drugs and ions. It is a smooth muscle of the urogenital tract, which, in the male, runs on to form the retractor of the penis. The rich adrenergic, cholinergic, serotoninergic and purinergic, in addition to the non-adrenergic, non-cholinergic (NANC) innervations make it a unique and dynamic smooth muscle preparation for experimental pharmacology.
The anococcygeus muscle has a dense sympathetic innervation, which comprises approximately 60% of the total innervation. The cellular mechanisms of contraction and relaxation of the anococcygeus smooth muscle are not deeply understood. The main signal for contraction of smooth muscle is an increase in the levels of free cytosolic calcium, which occurs as a result both of calcium entry from the extracellular space and calcium release from intracellular stores. Since the mechanisms which regulate calcium influx and intracellular calcium release are a controversial theme in previous studies, this review will focus on these mechanisms. In this mini-review, we propose to approach the calcium entry, analyzing the contribution of extracellular calcium, via voltage-operated channels, non-selective cationic channels and calcium capacitative influx. In addition, we will discuss the mechanisms of intracellular calcium release from sarcoplasmic reticulum sensitive to caffeine, α1-adrenoceptors agonists, ryanodine, and also calcium release from mitochondria. As reported by several authors, the α1- adrenoceptor subtypes, such as α1A, α1B can be classified in accordance to the sources of calcium. This mini-review will attempt to critically examine this assumption, and presents the understanding of the calcium signals in anococcygeus smooth muscle contraction.
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