Intracellular pH regulation and sperm motility in the European eel

Abstract

Sperm activation involves ion fluxes as well as a previous maturation in the seminal plasma, something which has not been studied in depth in marine fish species. pH and potassium are probably involved in sperm maturation and motility in the European eel, as indicated in previous studies. In this work, the absolute intracellular concentration of potassium in European eel sperm has been determined for the first time. In addition, the intracellular pH (pHi) of quiescent eel spermatozoa was determined by two methods (nigericin and null point) that gave similar results, 7.4-7.6. The natural pHi range of sperm samples in the quiescent stage was 7.4-8.0, with no evident relationship with sperm motility. However, a linear correlation was seen between sperm motility and the pH of the diluent or extracellular pH (pHe), as well as between the pHi and the pH of the diluent.The pHi change post-activation in seawater (ASW) depended on the initial pHe of the diluent medium. Activation with ASW induced an internal alkalinization of the cells when the sample had previously been diluted in a pHe<8.0; an acidification when pHe>8, and no pHi variation when pHe was 8.0. These experiments indicated that a careful selection of the diluents should be performed before measuring natural pHi changes in sperm cells. Thus, studies on the specific seminal plasma composition of marine fish species are necessary before studying their physiology. Furthermore, our study indicates that intracellular alkalinization is not a universal fact during sperm activation.

Role of Ion Channels in the Maintenance of Sperm Motility and Swimming Behavior in a Marine Teleost

“…In both FW and SW teleosts, different studies have shown that pH i is a key factor for the activation and maintenance of sperm motility, although the specific pH i changes required for sperm motility seem to vary among species [ 12 , 23 , 24 , 25 , 26 , 27 , 28 ]. To investigate whether seabream sperm motility also depends on the pH i , we performed motility assays using SW at pH 6 or 8, which was alkalized or acidified by 250 mM NH 4 Cl or 1.6 mM HCl, respectively, at ~25 s post activation.…”

“…To investigate whether seabream sperm motility also depends on the pH i , we performed motility assays using SW at pH 6 or 8, which was alkalized or acidified by 250 mM NH 4 Cl or 1.6 mM HCl, respectively, at ~25 s post activation. Previous studies in the catadromous European eel have shown that the pH i of spermatozoa is linearly dependent on the extracellular pH [ 12 ]. Our data showed that spermatozoa are not activated with SW at pH 6, whereas further intracellular alkalinization by NH 4 Cl triggers motility to the same extent as SW at pH 8 ( Figure 2 a).…”

“…Spermatozoa flagellar beating is controlled by pH sensitive axonemal dynein ATPases, which transduce chemical energy derived from ATP hydrolysis into the mechanical force necessary for the bending of flagella, and therefore the pH i contributes to the regulation of flagellar motility [ 38 , 39 ]. Some studies have shown that the pH i of fish spermatozoa varies in parallel with the extracellular pH (pH e ) [ 12 , 27 ] and, in some fish species, the pH e greatly affects the spermatozoon membrane potential regardless of the presence of Na + or K + in the external medium [ 40 ]. In many fish, as well as in mammals, alkalization of the pH i is required for motility activation [ 2 , 12 , 37 ], although in some fish species, such as eels, sperm can become motile with acidification, or even with no change of the pH i [ 12 , 26 ].…”

“…In most marine teleosts, sperm motility is triggered by exposure to the hyperosmotic SW and requires a surge of the [Ca 2+ ] i , and in some cases also of [K + ] i , which may be associated with the alkalization of the pH i [ 2 , 3 , 9 , 12 ]. These events activate the axonemal machinery directly, or through the Ca 2+ /calmodulin- or cAMP-dependent and -independent protein phosphorylation/dephosphorylation of axonemal proteins [ 2 , 13 ].…”

The effects of protein kinase A catalytic subunit on sperm motility regulation in Pacific abaloneHaliotis discus hannai

“…Their spermatozoa are quiescent in the testis and become motile after discharge into the aquatic environment (Nomura et al, 2005). It has been reported that the environmental factors such as temperature, osmotic pressure and pH could affect the percentage of motile sperm and sperm velocities (Dumorné et al, 2017;Pérez, Gallego, & Asturiano, 2020). The underlying regulation mechanism of sperm motility is absolutely important for the reproduction of aquatic animals.…”