Low seminal plasma buffering capacity corresponds to high pH sensitivity of sperm motility in salmonids

Abstract

The buffering capacity of air-equilibrated seminal plasma from the chinook salmon Oncorhynchus tshawytscha and steelhead Oncorhynchus mykiss were determined. Buffering capacities were significantly less at pH values at and above 7.3 than over the range 6.0 to 7.0 and were particularly low between pH values of approximately 7.5 to 8.5. The potential of chinook salmon sperm to become motile upon addition to water was highly sensitive to pH. Only very low levels of motility were demonstrable when sperm had been maintained at pH values at or below 7.5 prior to water exposure while maximum motility was noted when sperm had been maintained at pH values of about 8.0 and above. Thus, the potential for motility appeared especially sensitive to that pH range over which seminal plasma exhibited its lowest buffering capacity. Buffering capacity, protein content and inorganic phosphate content of chinook salmon and steelhead seminal plasmas were less than those of corresponding blood plasma or ovarian fluid suggesting that seminal plasma is unique among salmonid body fluids in its composition and low buffering capacity. This low buffering capacity is likely to facilitate the role of the sperm duct epithelium in semen pH regulation and thus, in the sperm's acquisition of the potential for motility upon exposure to water.