SECTION I CELLULAR PHYSIOLOGY Teakness and IgG A 43-year-old man presents symptoms that include weakness of the lower limbs. The patient complains of difficulty in climbing stairs.On examination he has dininished muscle stretch reflexes.There is a transient increase in ruscle power after maximal exercise.Electromyographic (EG)studies of this patient reveal a diminished amplitude of compound muscle action potentials.The compound muscle action potential is increased by over 200%in amplitude after vigorous exercise of the muscle studied.Single fiber muscle action potentials show significant hlock (i.e.,an action potential in the motor neuron fails to elicit an action potential in the muscle). After purified IgG fron this patient vas injected intravenously into a mouse. the mouse displayed signs similar to those of the patient.Studies of the neuromuscular junctions of the mouse's diaphragn revealed that endplate potentials recorded by microelectrodes in the muscle were greatly reduced.The frequency of miniature endplate potentials (rEPPs)in the IgG-injected mouse was similar to the EPP frequency in diaphragns fron control mice.However,when the diaphragns vere bathed in elevated K+(17.5 nl).the aEPP frequency in the diaphragns from coatrol mice increased dramtically,in contrast to a very slight increase in mEPP frequency in diaphragas fron IgG-injected mice.When diaphragms were treated with low doses of a calcium fomophore,the frequency of nEPPs greatly increased (this response was the sane in diaphragns from Igt-treated nice and from coatrol nice). Adrenal chromffin cells release their chromaffin granule coatents by exocytosis.by a process that resembles events in presynaptic nerve terminals. (Chromaffin cells can be patch clamped.When bovine adrenal chromaffin cells vere treated with IgG from the patient,the whole-cell Cat+current elicited by depolarization of the cells decreased dramatically to 0 aV (compared with control
SECTION I CELLULAR PHYSIOLOGY Weakness and IgG A 43-year-old man presents symptoms that include weakness of the lower limbs. The patient complains of difficulty in climbing stairs. On examination he has diminished muscle stretch reflexes. There is a transient increase in muscle power after maximal exercise. Electromyographic (EMG) studies of this patient reveal a diminished amplitude of compound muscle action potentials. The compound muscle action potential is increased by over 200% in amplitude after vigorous exercise of the muscle studied. Single fiber muscle action potentials show significant block (i.e., an action potential in the motor neuron fails to elicit an action potential in the muscle). After purified IgG from this patient was injected intravenously into a mouse, the mouse displayed signs similar to those of the patient. Studies of the neuromuscular junctions of the mouse's diaphragm revealed that endplate potentials recorded by microelectrodes in the muscle were greatly reduced. The frequency of miniature endplate potentials (mEPPs) in the IgG-injected mouse was similar to the mEPP frequency in diaphragms from control mice. However, when the diaphragms were bathed in elevated K+ (17.5 mM), the mEPP frequency in the diaphragms from control mice increased dramatically, in contrast to a very slight increase in mEPP frequency in diaphragms from IgG-injected mice. When diaphragms were treated with low doses of a calcium ionophore, the frequency of mEPPs greatly increased (this response was the same in diaphragms from IgG-treated mice and from control mice). Adrenal chromaffin cells release their chromaffin granule contents by exocytosis, by a process that resembles events in presynaptic nerve terminals. (Chromaffin cells can be patch clamped.) When bovine adrenal chromaffin cells were treated with IgG from the patient, the whole-cell Ca++ current elicited by depolarization of the cells decreased dramatically to 0 mV (compared with control
cells not treated with IgG).Nhen individual Ca++channels were studied by patch clanp,their properties were not altered by the treatnent with the patient's IgG. The patient was subjected to the following separate therapeutic interventions: (1)plasmapheresis,(2)imunosuppressive therapy,and (3)treatment with 4-aninopyridine (which blocks voltage-dependent Ktchannels).Each of these interventioes improved the patient's condition. 1.Why might depolarization-induced Ca currents in adrenal chromaffin cells be diminished by treating the cells with IgG from the patient? 2.What can you conclude from the observation that single Ca+tchannels were not affected by the patient's IgG? 3.What can you conclude from the observations that the frequency of mEPPs in diaphragms of control and IgG-injected nice was similar,but that the response to treating the preparation with elevated K*was so different (i.e..the frequency of nEPPs increased much more in the diaphragms of control mice)? 4.What can you conclude from the observation that the increase in frequency of mEPPs was sinilar in diaphragms from control and IgG-injected mice in response to treatment with the caleiun fomophore? 5.Why are mormal EPPs (evoked by motor neuron stimulation)reduced in size in diaphragms from IgG-injected nice compared vith those froa control mice? 6.Why should 'blocking"be observed in single-fiber EMG studies of the patient's muscles? 7.Why should the amplitude of the compound muscle action potential be reduced in respoase to voluntary contractions of a muscle (compared with a normal individual)? 8.Why should vigorous exercise of the involved musele result in a marked increase in the amplitude of the compound muscle action potential? 9.Why should muscular weakness he confined to the patient's lower limhs?
cells not treated with IgG). When individual Ca++ channels were studied by patch clamp, their properties were not altered by the treatment with the patient's IgG. The patient was subjected to the following separate therapeutic interventions: (1) plasmapheresis, (2) immunosuppressive therapy, and (3) treatment with 4-aminopyridine (which blocks voltage-dependent K+channels). Each of these interventions improved the patient's condition. 1. Why might depolarization-induced Ca currents in adrenal chromaffin cells be diminished by treating the cells with IgG from the patient? 2. What can you conclude from the observation that single Ca++channels were not affected by the patient's IgG? 3. What can you conclude from the observations that the frequency of mEPPs in diaphragms of control and IgG-injected mice was similar, but that the response to treating the preparation with elevated K+ was so different (i.e., the frequency of mEPPs increased much more in the diaphragms of control mice)? 4. What can you conclude from the observation that the increase in frequency of mEPPs was similar in diaphragms from control and IgG-injected mice in response to treatment with the calcium ionophore? 5. Why are normal EPPs (evoked by motor neuron stimulation) reduced in size in diaphragms from IgG-injected mice compared with those from control mice? 6. Why should "blocking" be observed in single-fiber EMG studies of the patient's muscles? 7. Why should the amplitude of the compound muscle action potential be reduced in response to voluntary contractions of a muscle (compared with a normal individual)? 8. Why should vigorous exercise of the involved muscle result in a marked increase in the amplitude of the compound muscle action potential? 9. Why should muscular weakness be confined to the patient's lower limbs?
ANSVER 1.The simplest explanation of this result is that IgG from the patient blocks the voltage-dependent Catt channels of the adrenal chronaffin cell plasma meabrane. These are the sane type of voltage-gated Ca++channels that are present in motor nerve terninals. 2.It appears that the block of the Cat+channels by the IgG is all-or-none. This implies that the IgG acts to reduce the number of activatable Ca++chanmels in the chromaffin cell plasna menbrane (and by inference,in the motor nerve terminal). 3.A major determinant of the frequency of nEPPs is the resting cytosolic Catt level in the notor nerve terminal.Thus the observation that the basal nEPP frequency is not affected by injecting IgG suggests that the IgG does not alter processes that set the level of Ca++in the nerve terminal eytosol.In diaphragns fron IgG-injected nice,the small increase in mEPP frequency in response to elevated extracellular K+is comsistent with the explanation in answer 2.If the IgG reduces the nurher of voltage-activatable Cat+channels,then fever Cat+chansels will open in respoase to K+-induced depolarization of nerve terminals in the diaphraom, the cytosolic Catt concentration in the nerve terninals will not rise so high in the Igt-treated diaphragm,and hence there vill be a smaller increase in mEPP frequency in the IgG-treated diaphragm. 4.The similar response of IgG-treated diaphragns and control diaphragns to the calciu iomophore suggests that it induces similar increases in intracellular Ca++in the merve terminals of the control and IgG-treated preparations and that the response to the elevated Ca++is simflar in both.This suggests that the IgG does not alter the events in the motor nerve terminal that occur after intracellular Catt rises. 5.If the patient's Igg blocks voltage-dependent Ca++channels in the motor nerve terminals,then an action potential in the notor neuron vill release fewer quanta of acetylcholine:this will result in saller EPPs
ANSWER 1. The simplest explanation of this result is that IgG from the patient blocks the voltage-dependent Ca++ channels of the adrenal chromaffin cell plasma membrane. These are the same type of voltage-gated Ca++ channels that are present in motor nerve terminals. 2. It appears that the block of the Ca++ channels by the IgG is all-or-none. This implies that the IgG acts to reduce the number of activatable Ca++ channels in the chromaffin cell plasma membrane (and by inference, in the motor nerve terminal). 3. A major determinant of the frequency of mEPPs is the resting cytosolic Ca++ level in the motor nerve terminal. Thus the observation that the basal mEPP frequency is not affected by injecting IgG suggests that the IgG does not alter processes that set the level of Ca++ in the nerve terminal cytosol. In diaphragms from IgG-injected mice, the small increase in mEPP frequency in response to elevated extracellular K+ is consistent with the explanation in answer 2. If the IgG reduces the number of voltage-activatable Ca++ channels, then fewer Ca++ channels will open in response to K+-induced depolarization of nerve terminals in the diaphragm, the cytosolic Ca++ concentration in the nerve terminals will not rise so high in the IgG-treated diaphragm, and hence there will be a smaller increase in mEPP frequency in the IgG-treated diaphragm. 4. The similar response of IgG-treated diaphragms and control diaphragms to the calcium ionophore suggests that it induces similar increases in intracellular Ca++ in the nerve terminals of the control and IgG-treated preparations and that the response to the elevated Ca++ is similar in both. This suggests that the IgG does not alter the events in the motor nerve terminal that occur after intracellular Ca++ rises. 5. If the patient's IgG blocks voltage-dependent Ca++ channels in the motor nerve terminals, then an action potential in the motor neuron will release fewer quanta of acetylcholine; this will result in smaller EPPs
6.This blocking is best explained by assuming that the diminished size of the EPPs in the patient results in some EPPs not having sufficient strength to depolarize the muscle fiber to the threshold for firing an action potential.This hardly ever happens in a nornal individual. 7.The likely explanation for the low-amplitude action potential is that some of the individual ruscle fibers that are comtributing to the compound action potential fail to fire action potentials in response to impulses in the motor axons that innervate them.As a result,fever of the patient's muscle fibers will contribute to each compound nuscle action potential. 8.Following vigorous exercise.nost of the nerve terminals in the muascle being studied have been activated at high frequency.For a period after the high-frequency stimulation.the motor nerve terminals will release more acetylcholine in response to a normal depolarization by the sane processes that occur in post-tetanic potentiation.The cellular bases of post-tetanic potentiation are not completely understood. 9.Plasmapheresis will reduce the concentration of circulating antibodies that are blocking Ca++channels and thus will increase the fraction of Ca++chanmels in the prejunctional motor nerve terminals that behave normally.Imunosuppressive therapy should have the sane effect.4-Aminopyridine will block voltage-gated K+ in the potor nerve terminal.As a result,the depolarixation of the nerve terninal caused by the action potential will be prolonged.Thus activation of the voltage-dependent Catt chanmels will be prolonged Influx of Catt into the nerve terminal will increase and thereby will release more acetylcholine
6. This blocking is best explained by assuming that the diminished size of the EPPs in the patient results in some EPPs not having sufficient strength to depolarize the muscle fiber to the threshold for firing an action potential. This hardly ever happens in a normal individual. 7. The likely explanation for the low-amplitude action potential is that some of the individual muscle fibers that are contributing to the compound action potential fail to fire action potentials in response to impulses in the motor axons that innervate them. As a result, fewer of the patient's muscle fibers will contribute to each compound muscle action potential. 8. Following vigorous exercise, most of the nerve terminals in the muscle being studied have been activated at high frequency. For a period after the high-frequency stimulation, the motor nerve terminals will release more acetylcholine in response to a normal depolarization by the same processes that occur in post-tetanic potentiation. The cellular bases of post-tetanic potentiation are not completely understood. 9. Plasmapheresis will reduce the concentration of circulating antibodies that are blocking Ca++ channels and thus will increase the fraction of Ca++ channels in the prejunctional motor nerve terminals that behave normally. Immunosuppressive therapy should have the same effect. 4-Aminopyridine will block voltage-gated K+ in the motor nerve terminal. As a result, the depolarization of the nerve terminal caused by the action potential will be prolonged. Thus activation of the voltage-dependent Ca++ channels will be prolonged. Influx of Ca++ into the nerve terminal will increase and thereby will release more acetylcholine