Membrane potential regulates sea urchin sperm adenylylcyclase

Area of Science:

• Reproductive Biology
• Cellular Physiology
• Biochemistry

Background:

• Sea urchin sperm adenylyl cyclase (AC) was previously thought to be regulated by G proteins.
• Sperm activation and acrosome reaction involve changes in membrane potential and cyclic AMP (cAMP) levels.
• Existing hypotheses suggest intracellular calcium and pH increases stimulate sea urchin sperm AC.

Purpose of the Study:

• To investigate whether membrane potential modulates sea urchin sperm adenylyl cyclase (AC) activity.
• To determine the role of membrane potential in regulating cAMP production during sperm activation.

Main Methods:

• Hyperpolarization of *Lytechinus pictus* sperm using valinomycin or ion-free artificial sea water (ASW).
• Measurement of intracellular cAMP levels under various ionic conditions (low K+, low Na+, absence of Ca2+).
• Treatment with IBMX (phosphodiesterase inhibitor) to enhance cAMP levels.
• Speract stimulation of *Strongylocentrotus purpuratus* sperm in ion-depleted ASW.

Main Results:

• Hyperpolarization of *L. pictus* sperm significantly increased intracellular cAMP levels (2.2- to 5.8-fold), even without extracellular Ca2+.
• These increases were potentiated by IBMX and occurred under conditions where intracellular pH and Ca2+ could not rise.
• Speract-induced cAMP increases in *S. purpuratus* sperm under ion-depleted conditions were attributed to hyperpolarization, as they were independent of pH and Ca2+.

Conclusions:

• Sea urchin sperm adenylyl cyclase (AC) is modulated by membrane potential.
• Membrane potential directly influences cAMP production in sperm, independent of intracellular Ca2+ or pH.
• This finding challenges previous models of AC regulation in sea urchin sperm.