Differential effects of manganese and magnesium on two types of slowly adapting cutaneous mechanoreceptor afferent units in frogs

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Abstract
While perfusing a frog leg with various solutions, the mechanical responsiveness of slowly adapting (SA) cutaneous mechanoreceptor afferent units innervating the plantar surface of the hindfoot was examined. Perfusion with normal Ringer solutions enhanced the mechanical responsiveness of two types of the SA units, irregularly discharging Frog type I units and regularly discharging Frog type II units, in comparison with that by normal blood supply. On the other hand, when the perfusion was made with Ringer solutions containing Ca blockers, i.e., Mn and Mg, the response magnitudes of both units were reversibly and dose-dependently decreased. Ringer solutions containing 3 mM Mn or 20 mM Mg almost abolished the maintained responses of the Frog type I units to ramp stimulation of the skin, while Ringer solutions of 10 mM Mn significantly decreased those of the Frog type II units. It is suggested that Ca in the medium is a prerequisite to mechanoelectric transduction and the spike generation process at the terminal regions of these slowly adapting cutaneous mechanoreceptor afferent units. Findings that the Frog type I units were quantitatively more sensitive to these Ca blockers than the Frog type II units suggest that an additional process of higher Ca-sensitivity, e.g., chemical synaptic transmission, may be involved on the way from mechanoelectric transduction to the spike generation of the Frog type I units.