R. Nilles et al., ATP-INDUCED CYTOPLASMIC [CA2-CELLS - INVOLVED RECEPTOR AND CHANNEL MECHANISMS(]] INCREASES IN ISOLATED COCHLEAR OUTER HAIR), Hearing research, 73(1), 1994, pp. 27-34
Outer hair cells (OHC) of the mammalian cochlea are thought to preproc
ess the sound signal by active movements, which can be induced by elec
trical or chemical stimulation, e.g. depolarization evoked by high [K] or increased cytoplasmic [Ca2+]. Extracellular ATP has been found to
induce cytoplasmic [Ca2+] increases in OHC but involved mechanisms ha
ve not been elucidated. Cytoplasmic [Ca2+] was measured in non-enzymat
ically isolated single OHC using Fura-2 microspectrometry. Results, us
ing ATP/derivatives and other P-2-purinergic receptor (P(2)R) ligands,
as well as Ca2+-channel blockers and pertussis toxin, revealed severa
l signal transduction; pathways that increase cytoplasmic [Ca2+] in OH
C: a P-2-purinergic receptor (P(2)R) G-protein - effector (phospholipa
se C or an ion channel) system and a voltage-dependent Ca2+ channel. A
gonist potency studies denote a pattern analogous to that found in ske
letal muscle, i.e. ATP-alpha-S > ATP = 2-methyl-S-ATP much greater tha
n ADP > alpha,beta-methylene-ATP, but no activation by ADP beta F or U
TP, leaving a choice of P-2y or P(2z)R subtypes. The latter possibilit
y gained strength from calculations showing that up to 8% of ATP may h
ave formed the P(2z)R agonist ATP(4-) in the experimental medium. Expe
riments in Ca2+-free medium and with pertussis toxin revealed that the
main Ca2+ source was intracellular. Pertussis toxin did not affect [C
a2+] increase induced by carbachoI. Acetylcholine, administered a few
seconds before ATP, did not affect total cytoplasmic [Ca2+] increases.
Induced cytoplasmic [Ca2+] increases were high enough (> 500 nM at 50
mu M ATP/derivatives) to hyperpolarize the OHC membrane by opening K-channels and decreased little with time. Artifacts may have been caus
ed by the sustained Ca2+ levels, e.g. activation of proteases by the h
igh cytoplasmic [Ca2+]. Similar events in vivo may have pathological c
onsequences.