Calcium sensitivity of BK-type KCa channels determined by a separable domain

High conductance, Ca(2+)-activated (BK-type) K+ channels from mouse (mSlo) and Drosophila (dSlo) differ in their functional properties but share a conserved core resembling voltage-gated K+ channels and a tail appended to the core by a nonconserved linker. We have found that the channel subunit is physically divisible into these two conserved domains and that the core determines such properties as channel open time, conductance, and, probably, voltage dependence, whereas the tail determines apparent Ca2+ sensitivity. Both domains are required for function. We demonstrated the different roles of the core and tail by taking advantage of the functional differences between mSlo and dSlo. Heterologous pairing of cores and tails from mSlo and dSlo showed that single-channel properties were always characteristic of the core species, but that apparent Ca2+ sensitivity was adjusted up or down depending on the species of the tail. Thus, the tail is implicated in the Ca(2+)-sensing role of BK channels.

Blockade of cholinergic channels by chlorisondamine on a crustacean muscle

Details of the blocking action of chlorisondamine, a ganglionic nicotinic blocker, on the excitatory cholinergic currents of the spiny lobster gastric mill 1 (g.m.1) muscle are described. The steady-state block of cholinergic ionophoretic currents produced by chlorisondamine is strongly voltage-dependent. During a hyperpolarizing voltage step, a sequence of ionophoretic agonist pulses in the presence of chlorisondamine shows a large interpulse interaction manifested as a gradual diminution in response amplitude. The extent of diminution is dependent on the number of the pulse in a series and not on the duration of the interval between pulses. The slowly developing blockade is entirely dependent on agonist application. If agonist application is suspended for various time intervals following the development of a given blocked level in chlorisondamine, no recovery from the block is observed whether the rest interval is at the step potential or at more depolarized potentials. Recovery from a given blocked level can be observed if, during a depolarizing voltage step (to -60 mV) away from the potential at which the block was established (-140 mV), agonist is applied before return to the initial potential (-140 mV). Chlorisondamine produces a dose-dependent reduction in excitatory junctional current (e.j.c.) decay rate that is linear with chlorisondamine concentration and markedly dependent on voltage (approximately equal to 35 mV/e-fold change). Reduction in the amplitude of e.j.c.s occurred at concentrations of chlorisondamine that produced no detectable effect on e.j.c. decay. Alterations in e.j.c. amplitude showed time- and use-dependent aspects similar to those observed for ionophoretic currents. These results are discussed primarily in terms of a sequential model in which, following the binding of chlorisondamine to the opened ion channel, the channel can undergo a transition to a stable-blocked state that requires reactivation by agonist to become unblocked. This stable-blocked state is considered a closed-blocked channel.

Different types of blockade of crustacean acetylcholine-induced currents

The voltage dependence, concentration dependence, and agonist dependence of blocking and unblocking produced by anticholinergic agents on the ionophoretically activated cholinergic currents of the lobster gastric mill 1 (g.m.1) muscle were examined. Although the ionophoretic technique provides only qualitative information as to blocking mechanisms it is useful in revealing slow components of the blocking action of some drugs. At least two qualitatively different types of voltage-dependent block of the crustacean cholinergic currents were observed. For pempidine, mecamylamine and decamethonium (also chlorisondamine: Lingle, 1983), a slowly developing voltage-dependent block was produced that led to the formation of a stable-blocked state. Recovery from this stable-blocked state is largely dependent on subsequent application of agonist. In contrast, recovery from the voltage-dependent block produced by QX-222, atropine, procaine and curare either proceeds independently of agonist application or occurs too rapidly to be observed by the present methods. Blockade by hexamethonium reveals anomalous voltage dependence, being enhanced over some voltages and relieved with additional hyperpolarization. Blockade by trimetaphan is largely independent of membrane potential except at higher concentrations.

Comparison of excitatory currents activated by different transmitters on crustacean muscle. II. Glutamate-activated currents and comparison with acetylcholine currents present on the same muscle

The properties of glutamate-activated excitatory currents on the gm6 muscle from the foregut of the spiny lobsters Panulirus argus and interruptus and the crab Cancer borealis were examined using either noise analysis, analysis of synaptic current decays, or slow iontophoretic currents. The properties of acetylcholine currents activated in nonjunctional regions of the gm6 muscle were also examined. At 12 degrees C and -80 mV, the predominant time constant of power spectra from glutamate-activated current noise was approximately 7 ms and the elementary conductance was approximately 34 pS. At 12 degrees C and -80 mV, the predominant time constant of acetylcholine-activated channels was approximately 11 ms with a conductance of approximately 12 pS. Focally recorded glutamatergic extracellular synaptic currents on the gm6 muscle decayed with time constants of approximately 7-8 ms at 12 degrees C and -80 mV. The decay time constant was prolonged e-fold about every 225-mV hyperpolarization in membrane potential. The Q10 of the time constant of the synaptic current decay was approximately 2.6. The voltage dependence of the steady-state conductance increase activated by iontophoretic application of glutamate has the opposite direction of the steady-state conductance activated by cholinergic agonists when compared on the gm6 muscles. The glutamate-activated conductance increase is diminished with hyperpolarization. The properties of the marine crustacean glutamate channels are discussed in relation to glutamate channels in other organisms and to the acetylcholine channels found on the gm6 muscle and the gm1 muscle of the decapod foregut (Lingle and Auerbach, 1983).

Comparison of excitatory currents activated by different transmitters on crustacean muscle. I. Acetylcholine-activated channels

The properties of acetylcholine-activated excitatory currents on the gm1 muscle of three marine decapod crustaceans, the spiny lobsters Panulirus argus and interruptus, and the crab Cancer borealis, were examined using either noise analysis, analysis of synaptic current decays, or analysis of the voltage dependence of ionophoretically activated cholinergic conductance increases. The apparent mean channel open time (tau n) obtained from noise analysis at -80 mV and 12 degrees C was approximately 13 ms; tau n was prolonged e-fold for about every 100-mV hyperpolarization in membrane potential; tau n was prolonged e-fold for every 10 degrees C decrease in temperature. Gamma, the single-channel conductance, at 12 degrees C was approximately 18 pS and was not affected by voltage; gamma was increased approximately 2.5-fold for every 10 degrees C increase in temperature. Synaptic currents decayed with a single exponential time course, and at -80 mV and 12 degrees C, the time constant of decay of synaptic currents, tau ejc, was approximately 14-15 ms and was prolonged e-fold about every 140-mV hyperpolarization; tau ejc was prolonged about e-fold for every 10 degrees C decrease in temperature. The voltage dependence of the amplitude of steady-state cholinergic currents suggests that the total conductance increase produced by cholinergic agonists is increased with hyperpolarization. Compared with glutamate channels found on similar decapod muscles (see the following article), the acetylcholine channels stay open longer, conduct ions more slowly, and are more sensitive to changes in the membrane potential.

A glutamate-activated chloride conductance on a crustacean muscle

A glutamate-activated inhibitory response on a 'crustacean neuromuscular preparation that receives cholinergic excitatory innervation is described. Glutamate produced a dose-dependent conductance increase to C1-ions. The response was mimicked by ibotenic acid, but not by quisqualic acid, and was blocked by picrotoxin.