Showing papers by "Edson X. Albuquerque published in 1979"

Mode of Action of Quinacrine on the Acetylcholine Receptor Ionic Channel Complex

Abstract: The effect of quinacrine was studied on neuromuscular transmission in frog sartorius and rat soleus muscles and on the binding of ligands to the electric organ of Torpedo ocellata. Quinacrine (30-200 µM) blocked neuromuscular transmission in both muscles, and inhibited the carbamylcholine-induced membrane depolarization at the endplate. The most pronounced effect of quinacrine was on the acetylcholine (ACh)-receptor mediated postsynaptic conductance. It reduced the endplate current (EPC) peak amplitude, without marked departure from linearity of the current voltage relationship, and it shortened the EPC rise time. The rate of decay of the EPC was also altered by quinacrine, becoming less voltage-dependent at concentrations of about 5 µM and completely voltage-independent at drug concentrations of 30-100 µM. Biochemical studies on membranes from electric organ of the electric ray Torpedo ocellata revealed that quinacrine inhibited the binding of [3H]ACh and [3H]H12-HTX to the membrane-bound ACh-receptor and its ionic channel, respectively. The inhibition constant (Ki) values were 7.4 µM and 14 µM, respectively. It is suggested that the mode of action of quinacrine on neuromuscular transmission is complex and reflects interaction with both the ACh-receptor and its ionic channel.

Voltage- and time-dependent actions of piperocaine on the ion channel of the acetylcholine receptor.

Abstract: The effects of the local anesthetic piperocaine were investigated on the endplate current (EPC) of frog sartorius muscles and on the binding of ligands to the acetylcholine (ACh) receptor and its ion channel in membranes from the electric organ of Torpedo ocellata. Piperocaine (10-100 µM) did not prevent action potential activity in nerve or muscle. However, these concentrations of piperocaine depressed reversibly the peak amplitude of EPCs in a dose-dependent manner without altering the EPC reversal potential. The current-voltage relationship obtained with short conditioning voltage durations preceding the EPC remained approximately linear at the piperocaine concentrations used. When the time during which the membrane potential was maintained preceding the EPC was lengthened from 10 to 500 msec in presence of piperocaine, the current-voltage relationship became markedly nonlinear, thus suggesting that there was more binding of the drug to the ACh-receptor ion channel complex. Both drug concentration and increasingly negative membrane potential augmented this time-dependent effect. At negative membrane potentials piperocaine also reversibly accelerated the rise and decay times of the EPC, while the EPC falling phase remained a single exponential function of time. The relationship between log of EPC decay time constant (τ) and membrane potential was linear in presence of piperocaine, and the slope progressively decreased and reversed its direction as piperocaine concentration was increased, with the maximum observed acceleration of τ being at 75 µM of drug. The effect of piperocaine on τ was voltage dependent but time independent. Piperocaine inhibited competitively [3H]perhydrohistrionicotoxin binding to the electric organ membranes, with an inhibition constant (Ki) of 0.44 µM; and noncompetitively [3H]ACh binding to its ACh-receptor with a Ki of 12.0 µM. These findings suggest that piperocaine has at least two separate actions at the ACh-receptorion channel complex. One is binding to open channels which causes concentration and voltage-dependent alteration of EPC time course and decreased EPC amplitude and voltage sensitivity of the EPC falling phase. Another is binding to a less well-defined site on the ACh receptor ion channel complex, an action that leads to a further depression of the peak amplitude of the EPC which is concentration, voltage and also time-dependent.

Tetraethylammonium: Voltage-dependent action on endplate conductance and inhibition of ligand binding to postsynaptic proteins

Abstract: Tetraethylammonium (Et4N+) ions depressed the amplitude and accelerated the decay rate of spontaneously occurring and nerve-evoked endplate currents (EPCs) in frog sartorius muscle. The relationship between peak EPC amplitude and membrane potential became nonlinear in the presence of 100 μM Et4N+, and with drug concentrations of 250 μM or greater the current-voltage relationship exhibited negative conductance in the hyperpolarized region. Et4N+ modified the exponential dependence of the EPC decay on membrane potential such that the decays between -150 and -50 mV were abbreviated and voltage independent but remained near control levels at more positive membrane potentials. The minimal effective concentration of Et4N+ for altering the EPC time course was 10, and maximal effects were attained with 100 μM. Little additional shortening in the EPC decay phase was detected on raising the drug concentration to 1000 μM. Acetylcholine noise analysis revealed a voltage-dependent reduction in the mean channel open time, which was comparable in magnitude to the shortening in the EPC decay, and a depression of single-channel conductance. In concomitant biochemical studies, Et4N+ was found to inhibit the binding of both [3H]acetylcholine and [3H]perhydrohistrionicotoxin to receptor-rich membranes from the electric organ of Torpedo ocellata with Ki values of 200 μM and 280 μM, respectively. These results suggest that Et4N+ interacts with both the acetylcholine receptor and its associated ionic channel. The voltage-dependent actions of Et4N+ are attributed to blockade of the ionic channel in closed as well as open conformation.

Junctional and extra functional aspects of inherited muscular dystrophy in chickens: development and pharmacology *

Abstract: The most consistent clinical sign of inherited muscular dystrophy in chickens is a progressive inability of the birds to right themselves when placed on their backs. This clinical manifestation of the disease, first seen in the original line of dystrophic chickens (line 304) at 2-3 weeks ex ovo, persists in a newer strain of dystrophic chickens (line 413) as the result of an outcrossing of the original line-304 dystrophic chicken. The muscular weakness that affects primarily the fast, white musculature, such as the posterior latissimus dorsi (PLD) muscle, appears to correlate with a decrementing response in the muscle to indirect stimu1ation.l As the disease progresses, the cable properties of the muscle and spontaneous transmitter release are altered. In the original line of dystrophic chickens (i.e., line 304), the specific membrane resistance (R,) and membrane capacitance (C,,,) of dystrophic PLD muscle were significantly greater than normal, and at a later time (11-12 weeks ex o v o ) , spontaneous transmitter release This paper details the temporal sequence of these and other electrophysiologic observations in muscles of the older line of normal (line 200) and dystrophic (line 304) chickens and compares them with electrophysiologic measurements in muscles of the newer line of normal (line 412) and dystrophic (line 413) chickens at selected times ex ovo. It became evident during the studies on the newer line of dystrophic chickens that drastic changes had occurred in the outcrossing of the older dystrophic chicken that now cast much doubt on the nature of the dystrophic condition in chickens. We also examined the effects of denervation on PLD muscles of both normal and dystrophic chickens in an effort to define the cause of dystrophies in chickens. A potential therapeutic approach to the dystrophies is presented, along with a discussion of the rationale for such treatment and of the basic membrane defect in muscular dystrophies in chickens.