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 hyperp olarization (hypoxic hyperpolarization), which is associated with a re duction in neuronal input resistance. The role of extra- and intracell ular Ca2+ ions in hypoxic hyperpolarization was investigated. The hypo xic hyperpolarization was significantly depressed by tolbutamide (100 mu M); moreover, the response was reversed in its polarity in medium c ontaining tolbutamide (100 mu M), low Ca2+ (0.25 mM), and Co2+ (2 mM), suggesting that the hypoxic hyperpolarization is mediated by activati on of both ATP-sensitive K+ (K-ATP) channels and Ca2+-dependent K+ cha nnels. The hypoxic depolarization in medium containing tolbutamide, lo w Ca2+, and Co2+ is probably due to inhibition of the electrogenic Na-K+ pump and concomitant accumulation of interstitial K+. Hypoxic hype rpolarizations 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 ther e is an optimal extracellular Ca2+ concentration required to produce t he hypoxic hyperpolarization. Bis-(o-aminophenoxy)-N,N,N',N'-tetraacet ic acid (BAPTA)-AM (50-100 mu M), procaine (300 mu M), or ryanodine (1 0 mu M) significantly depressed the hypoxic hyperpolarization, suggest ing that Ca2+ released from intracellular Ca2+ stores may have an impo rtant role in the generation of hypoxic hyperpolarization. The high-af finity calmodulin inhibitor -(6-amino-hexyl)-5-chloro-1-naphthalenesul fonomide hydrochloride (W-7) (5 mu M) completely blocked, whereas the low-affinity calmodulin inhibitor N-(6-aminohexyl)-1-naphthalenesulfon omide hydrochloride (W-5) (50 mu M) did not affect, the hypoxic hyperp olarization. The calmodulin inhibitor trifiuoperazine (50 mu M) also s uppressed the hypoxic hyperpolarization. In addition, calcium/calmodul in kinase II inhibitor esulfonyl)-N-methyl-L-tyrosyl]-4-phenyl-piperaz ine (KN-62) (10 mu M) markedly depressed the amplitude and net outward current of the hypoxic hyperpolarization without affecting the revers al potential. In contrast, neither the myosin Light chain kinase inhib itor naphthalene-1-sulfonyl)-1H-hexa-hydro-1,4-diazepin hydrochloride (ML-7) (10 mu M) nor the protein kinase A inhibitor omocinnamyl-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 gen eration of the hypoxic hyperpolarization. In conclusion, the present s tudy indicates that, in the majority of hippocampal CA1 neurons, the h ypoxic hyperpolarization is due to activation of both K-ATP channels a nd Ca2+ dependent K+ channels.