Das Belichtungspotential der isolierten Retina des Einsiedlerkrebses (Eupagurus bernhardus L.) in Abhängigkeit von den extrazellulären Ionenkonzentrationen

Summary

1.The action potential of the isolated retina of the hermit crab Eupagurus bernhardus L. resulting from exposure to light has been measured with external electrodes under constant stimulus conditions.

Three measurements of the retinal action potentials (RAP's) were taken to observe the changes of the RAP's quantitatively: a) The amplitude h _max of the maximum; b) the amplitude h _e of the plateau, measured at the end of the stimulus, for taking the shape quotient h _max /h _e ; c) the peak-amplitude-time t _max. 2.The RAP's of the retina in a standard physiological saline are compared with those of the retina in salines of different ionic composition while osmotic pressure and p _h were kept constant. 3.Increasing K⁺-concentration reduces the amplitude h _max of the RAP's gradually, which is zero at 500 mM K⁺/l. The peak-time t _max decreases with increasing K⁺-concentration up to 50 mM K⁺/l, whereas at higher concentrations it increases. In contrast to this fact h _max/h _e increases up to 50 mM K⁺/l and decreases above this concentration (Fig. 3). 4.Increasing Ca⁺⁺-concentration reduces h _max (zero above 350 mM Ca⁺⁺/l) and t _max. h _max/h _e rises up to a Ca⁺⁺-concentration of about 30 mM Ca⁺⁺/l; whereas at a higher Ca⁺⁺-concentration it decreases again (Fig. 6). When the Ca⁺⁺-concentration is very low the fall of the RAP is much slowed down and the plateau h _e extremly rises. In a Ca⁺⁺-free saline which contained 1 mM/l Ethylendiamintetraacetic acid (EDTA) the retina lost its irritability reversibly (Fig. 8). 5.The amplitude of the RAP's is augmented with increasing Mg⁺⁺-concentration up to 10–30 mM Mg⁺⁺/l and decreases above this concentration (Fig. 11). When the saline contains virtually no other cations but Mg⁺⁺ (367 mM/l) the amplitude of the RAP is small (20%) but not zero. 6.In a buffered isotonic NaCl-solution as well as in a saline in which all the Cl^--ions are substituted by SO₄ ^---ions the amplitude of the RAP's is higher but the shape of the RAP's is changed in the same way as in other solutions with very low concentrations of Ca⁺⁺. 7.All the changes of the RAP's described so far are reversible. 8.Even when the retina is kept in a salt solution containing sodium in a very low concentration (ca. 3–5 mM/l, the sodium substituted by choline⁺-ions) for 5 hours the amplitude h _max of the RAP does not change significantly but the shape: the peaktime t _max is longer, h _max/ h _e is much greater. Afterwards when the retina is brought into standard saline again, the amplitude h _max increases, t _max remains almost unchanged and h _max/h _e , decreases strongly (Fig. 17). 9.Substitution of all the Na⁺-, Ca⁺⁺- and Mg⁺⁺-ions by choline⁺-ions results in a decrease of the amplitude h _max, a lengthening of t _max and a small increase of h _max/h _e . 10.Substitution of all the NaCl by glucose decreases the amplitude h _max, lengthens t _max very much and decreases the value of h _max/h _e but little. Afterwards, when the retina is brought into standard saline again the effect of the glucose solution on the amplitude h _max is only little reversible: h _max increases very little, t _max decreases strongly and h _max/h _e increases (Fig. 21). 11.With increasing external K⁺-concentration the resting potential decreases. The changes of the resting potential cause the changes in the shape of the RAP's. 12.The presence of a small concentration of Ca⁺⁺-ions outside of the cell membrane is obviously necessary for the ability of the cell membrane of the photoreceptor to increase its ionic permeability consequent to stimulation by light. Above a Ca⁺⁺-concentration of about 1 mM/l the raise of permeability of the cell membrane during illumination is smaller with increasing Ca⁺⁺-concentration. The velocity of the changes in permeability is augmented, especially of those changes concerned with the fall of the RAP. The effect of Mg⁺⁺-ions is somewhat similar to that of Ca⁺⁺-ions, but much weaker. 13.The changes of the RAP are mainly determined by the low Ca⁺⁺-content when the retina stays in the NaCl- or sulfate saline. 14.Choline⁺-ions probably greatly increase the raise of permeability of the cell membrane for the divalent cations Ca⁺⁺ and Mg⁺⁺ during excitation. 15.It is suggested that under normal conditions in Eupagurus the amplitude of the RAP is determined primarily by the Na⁺-concentration gradient over the receptor cell membrane. But also the divalent cations Ca⁺⁺ and Mg⁺⁺ contribute to the amplitude of the RAP, especially after the retina has been treated with choline chloride.