containing solution (to competitively block Ca2+ influx) caffeine had no effect when added after complete inhibition, but when added during partial inhibition of responses, the Mg2+-induced inhibition was transiently reversed or halted. This suggests that Ca2+ influx was a prerequisite for the action of caffeine. 4. Ryanodine (1 uM) increased the responses of Merkel cell receptors to mechanical stimulation by 7-60% (mean+ S.E.M., 32 + 10 9%, n = 5, P < 0 05) but had no effect on St II receptor responses.5. The Ca2+-induced Ca2+ release (CICR) inhibitor procaine inhibited St I receptor responses in a concentration-dependent manner. Near-maximal inhibition was attained with 100,uM procaine. In four St I units, mean responses were depressed to 25 % of control values. When both procaine (100 uM) and caffeine (10 mM) were introduced together, no net effect was seen. The responses of St II receptors were little affected by up to 100 /SM procaine superfusion. 6. It is concluded that the mechano-electrical transduction process in St I receptors (but not St II) includes a CICR pathway. Taken with previous findings on the role of Merkel cells, it is likely that CICR is occurring in the Merkel cells.

The sensory innervation of the papilla incisiva in the hard palate of the domestic goat was studied with light and electron microscopy, supplemented by electrophysiological studies of free nerve endings. The goat lacks incisor teeth. Grass and leaves are not bitten, but pulled off by pressing them between the tongue and papilla incisiva. Thus, the masticatory mucosa is subject to particularly heavy mechanical loads requiring functional specialization of the horny epithelium in the form of thickening, i.e., the papilla incisiva and 12-14 pairs of rugae palatinae. A thin layer of firm connective tissue (lamina propria) attaches the mucosa to the periost of the hard palate. Sensory nerve fibers were found most abundantly in the papilla incisiva. Their number decreased drastically in aboral direction. A section through the first four rugae palatinae contains only about 10% of the number of free nerve endings found in the same area of mucosa from the papilla incisiva. Four types of sensory nerve endings were found. Free nerve endings were seen ubiquitously in the epithelium and superficial layer of the lamina propria. Merkel nerve endings were found in the bases of the epithelial thickenings in the papilla incisiva and rugae palatinae. Few Ruffini corpuscles were found in the deeper layer of the lamina propria, while lamellated corpuscles were seen just below the basement membrane of the epithelial pegs. Thus, a variety of sensory nerve endings were found in the hard palate, especially in those areas that are in close contact with the tongue during chewing of food. This rich innervation suggests an important role in monitoring the mechanical properties of food. Recordings were made from cell bodies supplying these terminals. Classic low-threshold, slowly adapting responses were observed in Ass afferent populations. This activity was probably mediated by Merkel type endings. Alternately, high-threshold and suprathreshold responses obtained from Adelta category afferents were likely to be nociceptive. In support of this, threshold and suprathreshold sensitization was observed following injection of serotonin into the receptive field of Adelta populations. This activity was likely to be derived from the aforementioned free nerve endings.

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An isolated, functioning sinus hair preparation was developed to investigate cytoplasmic Ca2+ concentrations in intact Merkel cells using microfluorimetric techniques. Intracellular Ca2+ levels were monitored by means of photon counters in small groups of Merkel cells loaded with the calcium fluorescent indicators fura-2 or fluo-3. Mechanical stimulation of Merkel cells with fine glass rods resulted in small transient increases in intracellular Ca2+ levels (by about 20%) in the group of Merkel cells around the stimulating probe. A rise in Ca2+ is presumed to be essential for the postulated synaptic transmission to the afferent nerve terminal. Depolarization with a high concentration of potassium chloride (100 mM) caused increases in intracellular Ca2+ concentrations in Merkel cells (by about 70%) only in the presence of extracellular Ca2+, indicating an influx of Ca2+ through voltage-gated channels. The Ca2+ response was abolished neither by (+)-BayK8644 nor omega-conotoxin, suggesting that the Ca2+ channels are different from the classical L- or N-type channels. Extracellular application of ATP (10 microM to 5 mM) caused dose-dependent increases in intracellular Ca2+ levels in Merkel cells of up to sevenfold from the basal level of about 100 nM. Similar responses to ATP were also measured during superfusion with Ca(2+)-free medium, suggesting intracellular stores as the main Ca2+ source. Pre-incubation of Merkel cells with the purinoceptor antagonist suramin (100 microM) for 30 min reduced the Ca2+ responses to ATP by about 50% compared with control conditions. In conclusion, the results have demonstrated that a rise in intracellular Ca2+ in Merkel cells can be evoked by mechanical stimulation, membrane depolarization and chemical stimulation by ATP. These observations strongly suggest a possible contribution of Ca2+ to the normal responsiveness of Merkel cell mechanoreceptors, in turn supporting the hypothesis that Merkel cells are involved in the mechano-electric transduction process in sinus hair type I mechanoreceptors.

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