Ca channel currents were studied in freshly dispersed bovine adrenal glomerulosa cells to better understand the control of aldosterone secretion by extracellular K concentration (K.) and angiotensin II (All). The whole-cell variation of the patch voltage clamp technique was used. Two types of Ca channels were found. One type is similar to the "T-type" Ca channels found in many excitable cells. These channels deactivate slowly (x 7 ms at -75 mV) and inactivate rapidly during strong depolarizations. The second channel type activates and inactivates at more positive potentials than the T-type Ca channels and deactivates rapidly.

SUMMARY1. Ca channels were studied in the GH4C1 clonal cell line derived from rat anterior pituitary cells. The whole-cell variation of the patch-electrode voltage-clamp technique was used.2. Two types of Ca channels were found. One type ('slowly inactivating' channels) is insensitive to changes in holding potential, does not inactivate during test pulses lasting several seconds, and deactivates very quickly upon repolarization. For holding potentials <-40 mV, a second type of Ca channel is available for opening. This population ('transient' channels) differs from the first type in that it activates at more negative potentials, inactivates rapidly with either Ca or Ba as the charge carrier, deactivates about 10 times more slowly upon repolarization, and is less selective for Ba over Cs.3. Nimodipine preferentially blocks the slowly inactivating channels. Block of these channels is time-and voltage-dependent, such that block is maximized by long depolarizations.4

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SUMMARY1. The Ca2+ channel blocker D-600 (methoxyverapamil) paralyses single muscle fibres of the frog: fibres exposed to the drug at 7 0C give a single K+ contracture after which they are paralysed, unable to contract in response to electrical stimulation or further applications of K+.2. Paralyzed fibres contract in response to caffeine and have normal resting potentials and action potentials.3. Fibres treated with D-600 at 22 0C are not paralysed. Paralysed fibres warmed to 22 0C recover contractile properties: they twitch and give K+ contractures. Other workers have shown that D-600 blocks a Ca2+ channel at room temperature; thus, the paralytic action of D-600 is probably mediated by a different membrane protein, perhaps a different Ca2+ channel from that blocked at room temperature.4. These results suggest that the binding of D-600 can disrupt the mechanism coupling electrical potential changes across the T membrane to Ca2+ release from the sarcoplasmic reticulum.

The selectivity of fiber-cell membranes and surface-cell membranes in the frog lens is examined using a combination of ion substitutions and impedance studies. We replace bath sodium and chloride, one at a time, with less permeant substitute ions and we increase bath potassium at the expense of sodium. We then record the time course and steady-state value of the intracellular potential. Once a new steady state has been reached, we perform a small signal-frequency-domain impedance study. The impedance study allows us to separately determine the values of inner fiber-cell membrane conductance and surface-cell membrane conductance. If a membrane is permeable to a particular ion, we presume that the conductance of that membrane will change with the concentration of the permeant ion. Thus, the impedance studies allow us to localize the site of permeability to inner or surface membranes. Similarly, the time course of the change in intracellular potential will be rapid if surface membranes are the site of permeation whereas it will be slow if the new solution has to diffuse into the intercellular space to cause voltage changes. Lastly, the value of steady-state voltage change provides an estimate of the lens' permeability, at least for chloride and potassium. The results for sodium are complex and not well understood. From the above studies we conclude: (a) surface membranes are dominated by potassium permeability; (b) inner fiber-cell membranes are permeable to sodium and chloride, in approximately equal amounts; and (c) inner fiber-cell membranes have a rather small permeability to potassium.