INTRACELLULAR CALCIUM INCREASES WITH HYPERACTIVATION IN INTACT, MOVING HAMSTER SPERM AND OSCILLATES WITH THE FLAGELLAR BEAT CYCLE

At some time before fertilization, mammalian sperm undergo a change in movement pattern, termed hyperactivation. There is evidence that hype ractivation offers an advantage to sperm for detaching from the oviduc tal mucosa, for penetrating viscoelastic substances in the oviduct, an d for penetrating the zona pellucida. Hyperactivation is known to requ ire extracellular calcium, but little else is known about the mechanis ms by which calcium affects sperm movement. The calcium-sensitive fluo rescent dye indo-1 was used to follow intracellular calcium levels ([C a2+]i) in individual moving sperm. Sperm were loaded with 10 muM of th e acetoxymethyl ester form of the dye and then rinsed. The dye was exc ited at 340 nm by using a filtered xenon stroboscope, and images at th e 405-nm and 490-nm excitation maxima were simultaneously digitized at 30 per sec for 2.1 sec. [Ca2+]i was significantly higher in the acros omal and postacrosomal regions of the head and in the flagellar midpie ce (the principal piece could not be measured) in hyperactivated than in nonhyperactivated sperm (P < 0.0001). [Ca2+]i oscillations were det ected in the proximal half of the midpiece that were identical in freq uency to the flagellar-beat-cycle frequency in 12 of 17 hyperactivated sperm (median, 3.5 Hz). Rapid [Ca2+]i, oscillations were also detecte d in the acrosomal and postacrosomal regions, as well as in the distal midpiece. Oscillations were not eliminated by dampening the flagellar bending with methyl cellulose. The [Ca2+]i oscillations detected in s perm are significantly more rapid than oscillations detected in other cell types.