The effects of hypoxia on slowly adapting type I (SAI) cutaneous mechanoreceptors in the cat and rat.

In whatever mammalian receptor system Merkel cells are found, they are always associated with a characteristic slowly adapting response. The role of Merkel cells in the transduction process of slowly adapting Type I cutaneous mechanoreceptors (SAI receptors or touch domes) of rats and cats was investigated by mechanical and electrical stimulation of SAI receptors and their afferent fibers in an O2-depleted environment. Circulatory hypoxia was produced either by ventilating animals with N2 or by recirculating venous blood around a limb. In both these experimental preparations, the results obtained were identical. For receptor failure to occur, it was found necessary to have an O2-depleted environment on the limb surface. This was achieved by passing N2 into a gas-tight polythene sock placed over the limb. Replacement of N2 within the polythene sock with O2 was sufficient to bring about receptor recovery, irrespective of arterial blood PO2 levels. There was an inverse linear relationship between receptor response and time when touch domes were stimulated with N2 around the limb. In contrast, the replacement of N2 around the limb with O2 produced an exponential increase in the response with time. Correlated with receptor failure was a significant reduction in the number of dense-cored vesicles normally found in the Merkel cell cytoplasm adjacent to the nerve ending innervating the cell. Receptor recovery was associated with a return in the number of dense-cored vesicles back to that found in control cells. Hypoxia had no effect on the level of electrical stimulation necessary to initiate an action potential in the afferent fiber, even though the response of SAI receptors to mechanical stimulation had ceased. The results indicate that Merkel cell dense-cored vesicles are necessary for the characteristic slowly adapting response of SAI mechanoreceptors and that this may be due to the secretion of a transmitter substance stored within the vesicles.