Increasing temperature (4-22°C) increases the Ca 21 concentration required for activation of mechanically skinned frog muscle fibers . The pCa required for 50% maximal force (pCa5o) was inversely proportional to absolute temperature. Assuming that relative force is directly related to fractional occupancy of the C2-binding sites on troponin that regulate force, the shift was consistent with a Gibbs free energy change of binding (AG) of about -7 .8 kcal/ mol. This is close to the AG for Ca 2+ binding to the calcium-specific sites on troponin C reported by others . Decreasing Mgt' from 1 mM to 60 NAM shifts the force-pCa curves at either 4 or 22°C to higher pCa, but the shift of pCaso with temperature over this range (0 .4 log units) was the same at low and high Mg t+ . Maximal force increased with temperature for the entire range 4-22°C with a Q,jo of 1 .41, and over the restricted range 4-15°C with a Quo of 1 .20. From the dual effects of temperature on Cat+ activation and maximal force, one would expect that force would respond differently to temperature change at high or low Cat+ . At high Cat+ , a temperature increase will lead to an increased force. However, at low to intermediate Ca 2+ levels (below the intersection of the forcepCa curves for the initial and final temperatures), steady state force should decrease with increasing temperature. The inverse responses should occur with a decrease in temperature. These responses are observed when temperature is changed by rapid solution exchange.

SUMNMARY1. The membrane resistance of isolated salivary glands was found to decrease in response to 5-HT. The change in resistance was calciumdependent.2. The resistance change of the apical membrane was found to be much greater than the change in resistance of the basal membrane.3. Potential responses under current-clamped conditions showed that one part of the biphasic response to 5-HT (attributed to an increase in chloride permeability) could be reversed and the other part (attributed to an increase in a potassium pump) could not.4. These observations have been incorporated into a model which, on evaluation, predicts all of the observed potential changes during the action of 5-HT. It suggests that the potential responses reflect changes in the internal chloride concentration produced by the calcium-dependent increases in chloride permeability.

The effects on the potential difference across isolated frog skin (R. catesbeiana, R. pipiens) of changing the ionic composition of the bathing solutions have been examined. Estimates of mean values and precision are presented for the potential changes produced by substituting other alkali metal cations for Na at the outside border and for K at the inside border. In terms of ability to mimic Na at the outside border of bullfrog skin, the selectivity order is Li > Rb, K, Cs; at the outside border of leopard frog skin, Li > Cs, K, Rb. In terms of ability to mimic K at the inside border of bullfrog and leopard frog skin: Rb > Cs > Li ~ Na. Orders of anion selectivity in terms of sensitivity of the potential for the outside border of bullfrog skin are Br ~ C1 > NOs > I > SO4, isethionate and of leopard frog skin are Br, C1 > I, NOs, SO4. An effect of the solution composition (ionic strength?) on the apparent Na-K selectivity of the outside border is described. The results of the investigation have been interpreted and discussed in terms of the application of the constant field equation to the Koefoed-Johnsen-Ussing frog skin model. These observations may be useful in constructing and testing models of biological ionic selectivity. I N T R O D U C T I O NIn the recent literature, models of selectivity for alkali metal cations have appeared (1, 2). Inasmuch as such suggestions appear to offer pathways toward a more fundamental understanding of the basis of bioelectric phenomena, an attempt to furnish data for testing the validity of the hypotheses is in order. Experimental work with the alkali metal cations has a long history (3, 4), but there is a scarcity of studies in which members of the series have been quantitatively compared under conditions formulated in the light of modern theories of bioelectric phenomena (but see Sjodin (5, 6) on frog skeletal muscle).The suitability of frog skin for such a study lies in two facts. It maintains a 749

The suitability of frog skin glands as a model for the study of secretory mechanisms in exocrine glands was explored. Periodic voltage clamp was used to determine continually the short-circuit current, chord conductance, and electromotive force of frog skin during neural and pharmacological activation of the skin glands. Both the chord conductance and the short-circuit current increased with glandular activation; the temporal dissociation of these increases suggests that there are at least two separate components to the secretory response. The sensitivity of the secretory electrical changes to changes in the ionic composition of the bathing solutions supports the notion of electrogenic chloride active transport as being basic to the activity of the exocrine glands.