The sperm of mice and men are streamlined cells, designed to move fast and deliver a payload. To maintain their Phelps-like speed through the long haul to the egg, sperm cells need energy. But even though sperm have all the equipment they need to turn fuel into energy, they don’t carry any fuel with them. Instead, they get their energy from the environment (read: lady parts).
Though many different fuels–glucose, lactic acid, and pyruvate–are present in the female genital tract, researchers in Germany recently discovered that only glucose can ramp up sperm speed. But this was only true when other fuel types aren’t present. Glucose is the main sugar source in the uterus, but not the vagina. The results by Nadja Mannowetz, Petra Wandernoth, and Gunther Wennemuth at Saarland University were published July 20 in the journal PLoS One. The results could one day be important for improving infertility treatments.
Throughout their short little lives, sperm rely on a variety of different energy sources. While in storage in the seminal vesicles, before ejaculation, sperm use fructose. But, once they’ve entered the vagina, sperm lose their male-derived source of energy and have to switch to using fuel provided by the female. In the vagina, the main source of energy is lactic acid–the same chemical your muscles produce during exercise, making them sore. Though we tend to think of lactic acid as a waste product, it can also be an energy source if burned in the mitochondria–the cell’s power producer–using oxygen. Lactic acid (also known as lactate) may even be the brain’s preferred source of energy.
If they make it through the vagina and past the cervix, the sperm still have a long way to go. Through the uterus. In the uterus, the environment changes from acidic to alkaline (or basic), and the energy source changes again: to glucose. Thankfully, this environment is just what gives the sperm the final burst of energy they need.
Using isolated mouse sperm in the lab, Nadja Mannowetz and colleagues tested three different energy sources for their effect on sperm speed. To estimate sperm speed, they measured how fast the sperm tail could beat when bathed in glucose, lactic acid, or pyruvate (another energy source present in the vagina).
They checked the speed of sperm tail beats right after adding the fuel (time 0), and at 20 and 60 minutes later. If all three fuels are added at once, sperm maintain an even 3.3 beats per second for the whole hour. The same is true if lactic acid and pyruvate are added together, or if either one is added alone.
But glucose, the main fuel in the uterus, is a different story. When added alone to the sperm, the beat speed increases from 3.3 to 6.4 beats per second at the 20 minute timepoint. At 60 minutes, the speed is similar to at 20 minutes (about 6.3 beats per second). Why does only glucose ramp up the sperm’s speed?
The answer wasn’t so much about the energy content of the fuel used. Instead, it was more about what that fuel source did to the sperm’s environment. As long as pyruvate or lactic acid are present, the environment is acidic. Adding glucose doesn’t change the pH (a measure of acidity) enough to make a difference. But when glucose is by itself, the pH rises and the environment becomes less acidic. This makes the sperm’s tail beat faster, because the enzymes used to make energy in the sperm work best at a higher pH. However, they don’t work at all without some kind of energy, like glucose, so simply raising the pH without adding a fuel source doesn’t work.
These experiments help us understand how the environment and sperm machinery work together to efficiently power a swimming sperm. The results of this study may be important in helping to improve treatments for infertility, like in vitro fertilization. They can inform scientists about what the best sperm environments might be, and may give some clues about what to look for if the sperm seem normal, but aren’t swimming as fast as they should.
Mannowetz N, Wandernoth PM, & Wennemuth G (2012). Glucose is a pH-Dependent Motor for Sperm Beat Frequency during Early Activation. PloS one, 7 (7) PMID: 22911736