Potassium increases intracellular calcium simulating progesterone action in human sperm

Area of Science:

• Human reproductive biology
• Sperm physiology
• Calcium signaling

Background:

• Progesterone and zona pellucida are known inducers of the human sperm acrosome reaction, mediated by increased cytosolic calcium.
• High concentrations of potassium ions (K+) have been observed to enhance the rate of acrosome reaction in human sperm in vitro.

Purpose of the Study:

• To determine if the effect of potassium ions (K+) on the human sperm acrosome reaction is mediated by an increase in intracellular calcium ([Ca2+]i).
• To compare the effects of K+ and progesterone (P) on intracellular calcium ([Ca2+]i) and investigate the involvement of ion channels.

Main Methods:

• Human sperm were collected using the swim-up method and capacitated overnight.
• Intracellular calcium ([Ca2+]i) levels were measured using the fluorescent indicator Fura 2.
• Sperm were incubated with varying concentrations of potassium chloride (K+) and/or progesterone (P), with and without EGTA or dihydropyridine derivatives.

Main Results:

• Incubation with potassium chloride (1.25-20 mM) resulted in a dose-dependent increase in intracellular calcium ([Ca2+]i), similar to that induced by progesterone (P).
• The increase in intracellular calcium ([Ca2+]i) by both K+ and P was inhibited by EGTA, a calcium chelator.
• K+-induced calcium changes were unaffected by dihydropyridine derivatives, while combined K+ and P treatment showed an additive effect on [Ca2+]i.

Conclusions:

• Potassium ions (K+) increase intracellular calcium ([Ca2+]i) in human sperm, contributing to the acrosome reaction.
• Human sperm plasma membranes appear to possess distinct calcium channels responsive to progesterone (P) and potassium ions (K+).