Energy Metabolism of Spermatozoa: III. Energy-Linked Uptake of Calcium Ion by the Mitochondria of Rabbit Epididymal Spermatozoa*†*Presented at the 30th Annual Meeting of The American Fertility Society, Hollywood, Florida, April 1974.†Supported by Public Health Service grants HD-06274 and GM-12202 and National Science Foundation grant GB-23063

Mitochondrial permeability transition pore (MPTP) desensitization increases sea urchin spermatozoa fertilization rate

Mitochondrial permeability transition pore (MPTP) is a protein complex whose opening promotes an abrupt increase in mitochondrial inner membrane permeability. Calcium signaling pathways are described in gametes and are involved in the fertilization process. Although mitochondria may act as Ca(2+) store and have a fast calcium-releasing mechanism through MPTP, its contribution to fertilization remains unclear. The work aimed to investigate the MPTP phenomenon in sea urchin spermatozoa and its role on the fertilization. Several pharmacological tools were used to evaluate the MPTP's physiology. Our results demonstrated that MPTP occurs in male gametes in a Ca(2+) - and voltage-dependent manner and it is sensitive to cyclosporine A. Additionally, our data show that MPTP opening does not alter ROS generation in sperm cells. Inhibition of MPTP in spermatozoa strongly improved the fertilization rate, which may involve mechanisms that increase the spermatozoa lifespan. The present work is the first report of the presence of a voltage- and Ca(2+) -dependent MPTP in gametes of invertebrates and indicates MPTP opening as another evolutionary feature shared by sea urchins and mammals. Studies about MPTP in sea urchin male gametes may contribute to the elucidation of several mechanisms involved in sperm infertility.

Ca2+-stores in sperm: their identities and functions

Intracellular Ca2+ stores play a central role in the regulation of cellular [Ca2+](i) and the generation of complex [Ca2+] signals such as oscillations and waves. Ca2+ signalling is of particular significance in sperm cells, where it is a central regulator in many key activities (including capacitation, hyperactivation, chemotaxis and acrosome reaction) yet mature sperm lack endoplasmic reticulum and several other organelles that serve as Ca2+ stores in somatic cells. Here, we review i) the evidence for the expression in sperm of the molecular components (pumps and channels) which are functionally significant in the activity of Ca2+ stores of somatic cells and ii) the evidence for the existence of functional Ca2+ stores in sperm. This evidence supports the existence of at least two storage organelles in mammalian sperm, one in the acrosomal region and another in the region of the sperm neck and midpiece. We then go on to discuss the probable identity of these organelles and their discrete functions: regulation by the acrosome of its own secretion and regulation by membranous organelles at the sperm neck (and possibly by the mitochondria) of flagellar activity and hyperactivation. Finally, we consider the ability of the sperm discretely to control mobilisation of these stores and the functional interaction of stored Ca2+ at the sperm neck/midpiece with CatSper channels in the principal piece in regulation of the activities of mammalian sperm.

Metabolic strategy of boar spermatozoa revealed by a metabolomic characterization

Metabolomic characteristics in boar spermatozoa were studied using [1,2-(13)C(2)]glucose and mass isotopomer analysis. In boar spermatozoa, glycolysis was the main pathway of glucose utilization producing lactate/pyruvate, whereas no gluconeogenesis was seen. Slight glycogen synthesis through the direct pathway and some incorporation of pyruvate into the Krebs cycle also took place. Neither RNA ribose-5-phosphate nor fatty acid synthesis from glucose occurred despite the detection of pyruvate dehydrogenase activity. In contrast to the known metabolic activities in dog sperm, boar spermatozoa have low levels of energy production and biosynthetic activities suggesting two different metabolic profiles for the two different phenotypes.

Calcium clearance mechanisms of mouse sperm

The spermatozoon is specialized for a single vital role in fertilization. Past studies show that Ca2+ signals produced by the opening of plasma membrane entry channels initiate several events required for the sperm to reach and enter the egg but reveal little about how resting [Ca2+]i is maintained or restored after elevation. We examined these homeostatic mechanisms by monitoring the kinetics of recovery from depolarizing stimuli under conditions intended to inhibit candidate mechanisms for sequestration or extrusion of Ca2+ from the cytosol. We found that the Ca2+-ATPase pump of the plasma membrane performs the major task of Ca2+ clearance. It is essential in the final stages of recovery to achieve a low resting [Ca2+]i. With immunomethods we found a approximately 130-kD plasma membrane Ca2+-ATPase protein on Western blots of whole sperm extracts and showed immunolocalization to the proximal principal piece of the flagellum. The plasma membrane Na+-Ca2+ exchanger also exports Ca2+ when [Ca2+]i is elevated. Simultaneous inhibition of both mechanisms of extrusion revealed an additional contribution to clearance from a CCCP-sensitive component, presumably sequestration by the mitochondria. Involvement of SERCA pumps was not clearly detected. Many aspects of the kinetics of Ca2+ clearance observed in the presence and absence of inhibitors were reproduced in a mathematical model based on known and assumed kinetic parameters. The model predicts that when cytosolic [Ca2+] is at 1 microM, the rates of removal by the Ca2+-ATPase, Na+-Ca2+-exchanger, mitochondrial uniporter, and SERCA pump are approximately 1.0, 0.35, 0.33, and 0 micromole l(-1) s(-1), rates substantially slower than those reported for other cells studied by similar methods. According to the model, the Na+-Ca2+ exchanger is poised so that it may run in reverse at resting [Ca2+]i levels. We conclude that the essential functions of sperm do not require the ability to recover rapidly from globally elevated cytosolic [Ca2+].

Investigations into the metabolism of human spermatozoa in relation to penetration capacity

Using a special chamber, the authors investigated the penetration capacity of spermatozoa of 120 sterile couples in both simple and crossed mucus penetration tests. Moreover, the glycolytic activity and the oxygen consumption of the ejaculates were biochemically determined in an artificial cervical mucus medium. A negative spermatozoan penetration was found to be associated with a markedly reduced aerobic glycolytic activity. It was for low-motility spermatozoa that no penetration was observed in cervical mucus, if the aerobic glycolysis was diminished, but the respiratory activity enhanced. Comparative investigations into the penetration capacity of spermatozoa in an artificial cervical mucus medium permit the detection of causes of male sterility, which exhibit metabolic regulation disturbances of spermatozoa.

Effect of prolactin on the calcium binding and/or transport of ejaculated and epididymal human spermatozoa

The prolactin concentration in human seminal plasma and in human epididymal cauda fluid was assessed by radioimmunoassay. The prolactin concentration in cauda plasma was found to be similar to that found in male blood serum (5.5 to 9.1 ng/ml) and significantly lower than the concentration of the hormone found in seminal plasma obtained either from euspermic (48 +/- 12 ng/ml) or from vasectomized volunteers (50 +/- 10.2 ng/ml) (P less than 0.001). Calcium binding and/or transport in ejaculated spermatozoa were found to be little (0.29 +/- 0.08 nmoles/10(8) cells) and dependent on a quickly saturable process. The addition of 200 ng of human prolactin/ml induced a 60% increase in this parameter, while 50 ng of prolactin/ml were ineffective. Epididymal human spermatozoa differ from ejaculated sperm cells in showing greater, time-dependent, calcium binding and/or transport under basal conditions (2.00 +/- 0.35 nmoles/10(8) cells/hour), and in being more susceptible to the stimulating action of prolactin (4.4 +/- 0.68 nmoles/10(8) cells/hour in the presence of 50 ng of prolactin/ml).