ATP and other purines derived from it are now known to have transmitter functions. ATP is present in synaptic vesicles of sympathetic nerves, along with norepinephrine, and is responsible for rapid excitatory potentials in smooth muscle. Adenosine receptors are widespread in the brain and in vascular tissue. Adenosine is largely inhibitory in the CNS, and inhibition of adenosine receptors is believed to underlie the stimulatory effects of caffeine.
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| A DISEASE CAUSING MUSCLE WEAKNESS |
| A 35-year-old woman noticed that she had difficulty in keeping her eyes open. She also had periods of double vision when her voice was indistinct and nasal and she had difficulty swallowing. Her physician suspected myasthenia, a disease of nerve-muscle conduction. The serum titer of anti-acetylcholine receptor antibodies, was measured and found to be elevated. |
| Comment. The patient was suffering from myasthenia gravis. This is a disease that manifests itself as weakness of voluntary muscles and is corrected by treatment with acetylcholinesterase inhibitors. It is caused by autoantibodies directed against the nicotinic acetylcholine receptor, which circulate in serum. Because of these autoantibodies, transmission of nerve impulses to muscle is much less efficient than normal. |
| Drugs that inhibit acetylcholinesterase increase the concentration of acetylcholine in the synaptic space, which compensates for the reduced number of receptors. Improvement in nerve-muscle conduction in response to endrophonium can be used as a diagnostic test but requires several precautions, and long-acting acetylcholinesterase inhibitors such as pyridostigmine can be used to treat the disease but corticosteroids are often effective. |
| Study of histamine in nerves is complicated by the large amounts that are present in mast cells. Histamine is found in a small number of neurons, mainly in the hypothalamus, although their projections are widespread throughout the brain. It has been shown to control the release of pituitary hormones, arousal, and food intake. Antihistamines designed to control allergies caused by release from mast cells act on the H1 receptor and tend to be sedative, suggesting that other central functions also probably exist. The histamine receptor
in the stomach is of the H2 class, therefore the H2 inhibitors, such as cimetidine and ranitidine, that are used to treat peptic ulcers have no effect on allergy.
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