A technique is described for the determination of the rate of uptake of glucose by the excised rat diaphragm preparation at intervals as short as 20 seconds. The rate is estimated from the fall in glucose concentration which occurs when a suitable medium flows over the muscle at constant rate. When insulin is added to the perfusion fluid, the rate of glucose uptake rises to a value about 50 per cent higher than that established before the introduction of insulin. Under the conditions described the change in uptake rate requires about 10 to 12 minutes to reach completion, being three-quarters complete in about 7 minutes. Although the major part of this delay must be ascribed to the time required for the washing out of insulin-free medium from the apparatus, and for the diffusion of insulin into the interstitial space, it is suggested that some of the delay may be due to processes occurring between the arrival of insulin at the fibre surface and the exertion of its characteristic effect on glucose uptake, although neither the time-course nor the nature of this induction phase is established.

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