The mechanisms responsible for the initial transport of immotile sperm from the testis into the epididymis are still poorly understood. We show here by electron microscopy and immunohistochemical approaches that the tunica albuginea of the human testis contains abundantly contractile elements. This tissue is also distinguished by extraordinarily high concentrations of cyclic GMP (cGMP)-dependent protein kinase I, known to mediate cGMP-dependent relaxation. Atrial natriuretic peptide (ANP) and the nitric oxide donor sodium nitroprusside (SNP) increased cGMP production in isolated strips of the tunica, and the enzymes involved could be demonstrated by affinity cross-linking and immunological techniques. Contractile cells as well as ectopic Leydig cells were identified as sites of nitric oxide synthase expression. Physiological studies revealed spontaneous contractions exclusively in regions near the rete testis. These contractions could be attenuated but not abolished by cGMP, SNP, and ANP. Remarkably, SNP reduced only the amplitudes, whereas ANP in addition decreased the frequency of these contractions. In contrast, noradrenaline-induced contractions, detectable in all parts of the capsule, could be abolished completely by SNP. These data, demonstrating complex contraction and relaxation activities, are indicative of a major physiological role of the tunica albuginea presumably related to testicular sperm transport.
