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The ability of sciatic nerve A fibres to conduct action potentials relies on an adequate supply of energy substrate, usually glucose, to maintain necessary ion gradients. Under our ex vivo experimental conditions, the absence of exogenously applied glucose triggers Schwann cell glycogen metabolism to lactate, which is transported to axons to fuel metabolism, with loss of the compound action potential (CAP) signalling glycogen exhaustion. The CAP failure is accelerated if tissue energy demand is increased by high-frequency stimulation (HFS) or by blocking lactate uptake into axons using cinnemate (CIN). Imposing HFS caused CAP failure in nerves perfused with 10 mM glucose, but increasing glucose to 30 mM fully supported the CAP and promoted glycogen storage. A combination of glucose and lactate supported the CAP more fully than either substrate alone, indicating the nerve is capable of simultaneously metabolising each substrate. CAP loss resulting from exposure to glucose-free artificial cerebrospinal fluid (aCSF) could be fully reversed in the absence of glycogen by addition of glucose or lactate when minimally stimulated, but imposing HFS resulted in only partial CAP recovery. The delayed onset of CAP recovery coincided with the release of lactate by Schwann cells, suggesting that functional Schwann cells are a prerequisite for CAP recovery.
Abstract: Addition of histamine (0.1 m M ) to guinea‐pig hippocampal slices causes a 20‐ to 30‐fold increase in the accumulation of cyclic AMP compared with basal levels. This accumulation represents a balance between cyclic AMP production by adenylate cyclase and cyclic AMP breakdown mediated by phosphodiesterase (PDE). However, brain tissues are known to contain several different PDE isozymes. To determine which are involved in this response to histamine, the effect of isozyme‐specific PDE inhibitors on cyclic AMP accumulation was examined in the hippocampus. MB 22948 (0.1 m M ), an inhibitor of PDEs I and II, had no significant effect on the response to either 1 μM or 0.1 m M histamine. SKF 94120 (0.1 m M ), a PDE III inhibitor, was also without effect in the presence of 1 μM histamine, although with 0.1 m M histamine, it caused a weak (1.25‐fold compared with control), but statistically significant, enhancement of cyclic AMP accumulation. However, both rolipram (0.1 m M ), a PDE IV inhibitor, and 3‐isobutyl‐l‐methylxanthine (0.1 or 1 m M ), an inhibitor of all forms of PDE, significantly increased cyclic AMP accumulation (2.8‐ to 6.5‐fold compared with controls), and the relative size of this effect decreased with increasing histamine concentration. It is concluded that PDE IV is the main PDE isozyme involved in cyclic AMP turnover in guinea‐pig hippocampal slices responding to histamine.
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