Role
of ion channels and membrane potential in the initiation of carp sperm
motility
Z. Krasznai, M. Morisawa, S. Morisawa, Z.T. Krasznai,
L. Trón, R. Gáspár, T. Márián-2003
Abstract:
The exposure of freshly spawned, immotile carp sperm
to hypoosmotic media triggers the initiation of calcium-dependent flagellar
motility. Intracellular calcium concentration has been thought to be the
critical component in motility initiation, possibly acting through a novel
signalling pathway. The sensitivity of sperm cells to changes of osmolality
of the environment raises the question whether a mechanoregulated
osmosensitive calcium pathway is involved in the activation mechanism of
carp sperm motility. The sperm cells are in a depolarized state in the
seminal plasma (ψ = –2.6 ± 3 mV) and they
hyperpolarize upon hypoosmosis-induced activation of motility (ψ = –29 ± 4 mV).
The intracellular sodium [Na+]i, potassium [K+]i
and calcium [Ca2+]i ion concentrations were determined
in quiescent cells, and at 20, 60 and 300 s after activation. The
[Na+]i and [K+]i of the
quiescent cells were similar to the [Na+]e and [K+]e
of the seminal plasma. Following hypoosmotic shock-induced motility, both
[Na+]i and [K+]i decreased to
one-fourth of the initial concentration. The [Ca2+]i
doubled at initiation of the motility of the sperm cells and remained
unchanged for 5 min. Bepridil (50–250 µM), a blocker of the Na+/Ca2+
exchanger, blocked carp sperm motility reversibly. Gadolinium, a blocker of
stretch-activated channels (10–20 µM), inhibited sperm motility in a
dose-dependent manner and its effect was reversible. Hypoosmotic shock
fluidized the membrane and gadolinium treatment made it more rigid in both
quiescent cells and hypotonic shock treated but immotile sperm cells. Based
on these observations, it is suggested that, besides the well-known function
of potassium and calcium channels, stretch-induced conformational changes of
membrane proteins are also involved in the sperm activation mechanism of
common carp.
(Department of Biophysics and Cell Biology,
University Medical School, P.O. Box 38, Debrecen 4012, Debrecen Nagyerdei
krt. 98, Hungary, e-mail: Krasznai@jaguar.dote.hu)