Absorption and distribution of xenobiotics.

Extrapolation of pharmacokinetic data between species has been simplified by the advent of more sensitive methods of analysis of chemicals in body tissues and by the capability of inexpensive computers to perform complex calculations. These new methods enable investigators to observe the rates at which target tissues reach equilibrium in different species and to develop mathematical models of these processes. The evaluation of physiological pharmacokinetics from classical or compartmental kinetics is improving the ability to project the long-term behavior of chemicals in body fluids and organs based on independently derived physical, chemical, and physiological constants obtained from simple chemical reactions, tissue culture experiments, or short-term animal studies. Accurate prediction of chemical behavior by such models gives support to hypothetical mechanisms of distribution and accumulation, while significant deviations from predicted behavior signal the existence of previously unsuspected pathways. These techniques permit the simulation of the impact of linear, nonlinear, and saturation kinetics on chemical behavior; the prediction of integrated tissue exposure; and the mapping of the sequence of alternate metabolic pathways that lead to toxicity or detoxification. The discussion will identify the research needs for improving extrapolations between species.

Motor nerve ending disorder in myasthenia gravis.

Mild myasthenia gravis patients were compared with normals with respect to the capacity of their motor nerve endings (MNEs) to generate a neostigmine-induced postactivation repetition (PAR). Dose-response analyses of PAR recorded from muscle electrically and by contractile measurement disclose a loss of this pharmacologic responsiveness in myasthenia. Since mild myasthenics transmitted nerve impulse trains of 20 to 200 Hz, as did normals, it was evident that PAR is transmitted insofar as it can be generated by MNEs. The dose-response analyses support this. These data indicate an MNE disorder in the disease.

Physostigmine inhibition of 3',5'-cyclic AMP phosphodiesterase from cat sciatic nerve.

This study was designed to determine whether cholinergic drug interaction with cyclic (c) AMP phosphodiesterase (PDE) might account for part of the effects of this class of drugs at the neuromuscular junction. The activity levels of both high- and low-affinity forms of cAMP PDE from cat sciatic nerve were examined for drug inhibition or activation. Of the cholinergic drugs tested, only physostigmine and Tacrine produced significant cAMP PDE inhibition. Physostigmine was 10 times more potent than theophylline and half as potent as SQ 20,009 (known PDE inhibitors) in inhibiting motor nerve cAMP PDE. Tacrine inhibited this enzyme at concentrations comparable with theophylline. None of the other drugs tested (diisopropylfluorophosphate, edrophonium, neostigmine, ecothiophate, carbachol or d-tubocurarine) produced significant changes in cAMP PDE activity. The inhibitory effects of physostigmine were shown to be pH independent over a range of 7.0 to 8.5. Kinetic studies indicated a mixed form of inhibition for physostigmine and Tacrine comparable with that seen for theophylline. These data indicate that the anticholinesterase activity of physostigmine and Tacrine do not adequately describe the facilitatory actions of these drugs at the motor nerve ending.

Characterization of the train-of-four response in fast and slow muscles: effect of d-tubocurarine, pancuronium, and vecuronium.

The in vivo cat soleus and gastrocnemius muscles were used to compare isometric contraction strength and the train-of-four (T4) response (2 Hz for 2 s) of two muscle types (fast and slow) during onset of competitive neuromuscular blockade in order to determine the extent of the correlation between twitch depression and T4 fade. Prior to drug administration the muscles that were studied differed significantly in that the T4 ratio was 1.0 in the gastrocnemius and only 0.87 in the soleus. Three competitive neuromuscular-blocking agents were compared: d-tubocurarine, pancuronium, and vecuronium. d-Tubocurarine was found to produce a close correlation between the degrees of twitch strength depression and T4 for both muscles. However, these muscles demonstrated significantly different ED50 values (105 micrograms/kg for gastrocnemius, 150 micrograms/kg for soleus). Pancuronium also produced a similar relationship between twitch strength depression and T4 decrement for each muscle. In this case, however, there was little difference in their ED50 values for twitch depression (11.5 micrograms/kg for gastrocnemius, 13 micrograms/kg for soleus). The effects of vecuronium were quite different from the other two muscle relaxants. Although vecuronium produced a comparable correlation between twitch tension and T4 fade in fast muscle, no such relationship was found to exist in slow muscle. Even when the twitch strength was blocked to 18% of control, the soleus T4 response was depressed to only 75% of control. These results highlight major differences among competitive neuromuscular-blocking agents and suggest multiple sites of action.

Cyclic nucleotides and neuromuscular transmission.

A review of the research on cyclic nucleotides and neuromuscular transmission suggests that cAMP is involved in the release of transmitter from motor nerve endings. Lipid-soluble derivations of cAMP cause depolarization of unstimulated nerve endings and prolong the after potentials of stimulated nerve endings. They also increase the frequency of miniature end plate potentials and increase the quantal content of stimulus evoked end plate potentials. Similar effects are produced by compounds that activate adenylate cyclase or inhibit phosphodiesterase. The responses to the derivatives of cAMP and activators of cyclase are enhanced by inhibitors of phosphodiesterase and prevented by compounds that block the flux of calcium into nerve endings. There is no evidence that suggests that cyclic nucleotides are involved in the postjunctional response to transmitter. Thus, it seems likely that cAMP is involved in the regulation of calcium in motor nerve endings and the exocytosis of transmitter. Additional study should expand our knowledge of neuromuscular transmission and contribute to an understanding of the functions of cyclic nucleotides in other synapses.

Effects of calcium on a cyclic nucleotide system in soleus motor nerve terminals.

The effects of increased extracellular calcium levels on the responses of the motor nerve terminal to agents that increased intracellular levels of cyclic AMP was studied on the in vivo cat soleus muscle preparation. The neural and muscle responses to intra-arterially administered NaF, an activator of adenylate cyclase, and dbcAMP progressively increased as blood calcium levels rose. These results provide additional support for the hypothesis of an interaction between calcium and cyclic AMP in motor nerve endings.

Effects of phenytoin on the cyclic nucleotide system in the motor nerve terminal.

The effects of phenytoin on the motor nerve terminal were evaluated on the in vivo cat soleus nerve muscle preparation. Phenytoin, 10 mg/kg, reduced the repetitive aftercharges in motor nerve endings due to tetanic conditioning. It also reduced the repetitive activity due to adenylate cyclase activation with NaF, or to exogeneous dibutyryl cyclic AMP. These effects of phenytoin could be reversed by administering theophylline, a phosphodiesterase inhibitor, or by increasing the extracellular concentration of calcium. The effects of phenytoin could also be reversed by 3-aminopyridine, but not by tetraethylammonium chloride. Verapamil, a calcium current antagonist, produced effects that were identical to phenytoin. It is concluded that phenytoin blocks a cyclic nucleotide-mediated calcium influx that is associated with transmitter release. This calcium flux also appears to control a slow potassium current that is responsible for post-tetanic hyperpolarization.

Azathioprine: effects on neuromuscular transmission.

The neuromuscular effects of azathioprine were examined in the in-vivo cat soleus muscle preparation. In concentrations ranging from 10 to 1,000 mug/kg, administered intra-arterially, the agent caused motor axons to fire repetitively and produced a dose-related increase in the force of contraction. The drug reversed neuromuscular blockage produced by d-tubocurarine and potentiated the neuromuscular blockade produced by succinylcholine. The effects of theophylline, a phosphodiesterase inhibitor, on neuromuscular transmission were identical to those produced by azathioprine. Using an in-vitro assay preparation, azathioprine was found to produce 50 per cent inhibition (IC50) of phosphodiesterase at a concentration of 2 X 10(-5) M. In the same preparation, theophylline had an IC50 of 1 X 10(-4) M. Neither agent in concentrations to 10(-2) M affected cholinesterase activity measured in vitro. It is concluded that the effects of azathioprine on neuromuscular transmission are due to inhibition of phosphodiesterase in the motor nerve terminal.

Effects of cyclic nucleotides on mammalian motor nerve terminals.

This research examined the effects of several cyclic nucleotides on in vivo cat soleus nerves and muscles. The reagents were administered by rapid close intra-arterial injection while electrical activity in single motor axons and contractile activity in the whole muscle were monitored. Cyclic N6-2'-O-dibutyryl adenosine 3',5'-monophosphate (dibutyryl cAMP) initiated bursts of action potentials in unstimulated axons. It also caused the occurrence of stimulus bound repetitive potentials in stimulated axons. It caused the muscle to undergo a series of rapid asynchronous contractions and potentiated the strength of stimulus-evoked contractions. Monobutyryl cAMP produced similar responses, but was less potent than dibutyryl cAMP. cAMP produced only a small, transient depression of neuromuscular transmission. There was no response to dibutyryl cyclic 3',5' guanosine monophosphate or sodium butyrate. None of these reagents affected denervated muscle. The results suggest that cAMP-like materials that can penetrate nerve membranes cause depolarization of motor nerve terminals, prolongation of the depolarization of the terminal initiated by an action potential and release of transmitter.

A role of cyclic nucleotides in neuromuscular transmission.

This research explored the possibility that cyclic nucleotides are part of the excitation-secretion sequence in mammalian motor nerve terminals. A series of reagents known to react with the enzymes that synthesize and degrade cyclic nucleotides or that are effectors of cyclic nucleotide actions were administered to in vivo cat soleus nerve-muscle preparations. The reagents were administered by rapid close intra-arterial injection while electrical activity in single motor axons and contractile activity of the muscle were monitored. NaF, an activator of adenylate cyclase, evoked bursts of action potentials in unstimulated axons and caused stimulus-bound repetitive activity in stimulated axons. It evoked vigorous asynchronous activity in the muscle and potentiated the force of muscle contraction. These effects are identical with those of cyclic N6-2'-O-dibutyryl adenosine 3':5'-monophosphate (dibutyryl cAMP). Prostaglandin E1 produced similar effects. Dithiobisnitrobenzoic acid and alloxan, inhibitors of adenylate cyclase, impaired neuromuscular transmission and prevented the effects of NaF, but they did not change the responses to dibutyryl cAMP. Theophylline, an inhibitor of phosphodiesterase, caused axons to respond repetitively to stimulation, but this activity had a different pattern from that produced by dibutyryl cAMP or NaF. Pretreatment with theophylline enhanced the responses to dibutyryl cAMP and NaF. Imidazole, an activator of phosphodiesterase, impaired neuromuscular transmission and prevented the effects of dibutyryl cAMP and NaF. Adenosine, an inhibitor of protein kinase, or verapamil, which inhibits calcium flux, impaired neuromuscular transmission and prevented the responses to dibutyryl cAMP, NaF and theophylline. These results are compatible with the hypothesis that cAMP is involved in the regulation of calcium flux and transmitter secretion in mammalian motor nerve terminals.

Streptozotocin treatment of streptozotocin-induced islet cell adenomas in rats.

Hypoglycemic rats bearing insulin-secreting islet-cell adenomas produced by the combined action of streptozotocin and nicotinamide were treated with streptozotocin. Antitumor response was demonstrated by elevation of blood glucose, reduction in plasma and tumor IRI, and histopathologic changes in the beta-cell neoplasm. The rodent tumor model may serve as a predictive system for selection and investigation of mechanisms of action of future antitumor agents to be used in the treatment of malignant insulinoma in man.