Mediation by intracellular calcium-dependent signals of hypoxic hyperpolarization in rat hippocampal CA1 neurons in vitro

In response to oxygen deprivation, CA1 pyramidal neurons show a hyperpolarization (hypoxic hyperpolarization), which is associated with a reduction in neuronal input resistance. The role of extra- and intracellular Ca2+ ions in hypoxic hyperpolarization was investigated. The hypoxic hyperpolarization was significantly depressed by tolbutamide (100 mu M); moreover, the response was reversed in its polarity in medium containing tolbutamide (100 mu M), low Ca2+ (0.25 mM), and Co2+ (2 mM), suggesting that the hypoxic hyperpolarization is mediated by activation of both ATP-sensitive K+ (K-ATP) channels and Ca2+-dependent K+ channels. The hypoxic depolarization in medium containing tolbutamide, low Ca2+, and Co2+ is probably due to inhibition of the electrogenic Na+-K+ pump and concomitant accumulation of interstitial K+. Hypoxic hyperpolarizations were depressed in either low Ca2+ (0.25 or 1.25 mM) or high Ca2+ (5 or 7.5 mM) medium (control: 2.5 mM), indicating that there is an optimal extracellular Ca2+ concentration required to produce the hypoxic hyperpolarization. Bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid (BAPTA)-AM (50-100 mu M), procaine (300 mu M), or ryanodine (10 mu M) significantly depressed the hypoxic hyperpolarization, suggesting that Ca2+ released from intracellular Ca2+ stores may have an important role in the generation of hypoxic hyperpolarization. The high-affinity calmodulin inhibitor N-(6-amino-hexyl)-5-chloro-1-naphthalenesulfonomide hydrochloride (W-7) (5 mu M) completely blocked, whereas the low-affinity calmodulin inhibitor N-(6-aminohexyl)-1-naphthalenesulfonomide hydrochloride (W-5) (50 mu M) did not affect, the hypoxic hyperpolarization. The calmodulin inhibitor trifiuoperazine (50 mu M) also suppressed the hypoxic hyperpolarization. In addition, calcium/calmodulin kinase II inhibitor 1-[N,O-bis(1,5-isoquinol-inesulfonyl)-N-methyl-L-tyrosyl]-4-phenyl-piperazine (KN-62) (10 mu M) markedly depressed the amplitude and net outward current of the hypoxic hyperpolarization without affecting the reversal potential. In contrast, neither the myosin Light chain kinase inhibitor 1-(5-iodonaphthalene-1-sulfonyl)-1H-hexa-hydro-1,4-diazepin hydrochloride (ML-7) (10 mu M) nor the protein kinase A inhibitor N-[2-(p-bromocinnamyl-amino)ethyl]-5-isoquinolinesulfonamide (H-89) (1 mu M) significantly altered the hypoxic hyperpolarization. These results suggest that clamodulin kinase II, which is activated by calmodulin, may contribute to the generation of the hypoxic hyperpolarization. In conclusion, the present study indicates that, in the majority of hippocampal CA1 neurons, the hypoxic hyperpolarization is due to activation of both K-ATP channels and Ca2+ dependent K+ channels.