Absence of desensitization in the response of the guinea pig sinus node to carbamylcholine.
The effects of pairs of consecutive exposures to carbamylcholine (CBL) on the automaticity of the guinea pig sinus node and the transmembrane potentials of transitional fibers were studied. The preparations were superfused with Tyrode solution at 35 degrees C. Transmembrane potentials were recorded by use of intracellular microelectrodes. CBL 10(-6) M significantly depressed the sinus node frequency but did not produce sinoatrial arrest during a 1-min superfusion. The amplitude of the action potential of transitional fibers decreased, in the absence of any depressant effect on the maximum diastolic potential and maximum velocity of development of phase 0. The duration of the action potential was markedly reduced. These effects were observed in both of two consecutive 1-min exposures to CBL. CBL 10(-6) M and 10(-4) M produced sinoatrial arrest within 3 min and 1 min, respectively. Neither the time to arrest nor the time to restart of spontaneous activity were modified by a second exposure to CBL. Also, CBL 10(-4) M slightly increased the maximum diastolic potential of transitional fibers in both consecutive exposures to the drug. In conclusion, the effects of CBL on sinus node automaticity and on transmembrane potentials of transitional fibers did not show signs of desensitization.
D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D002217:Carbachol / Q000494:pharmacology*; D006168:Guinea Pigs; D006323:Heart Arrest / Q000503:physiopathology; D066298:In Vitro Techniques; D008564:Membrane Potentials / Q000187:drug effects; D011976:Receptors, Muscarinic / Q000187:drug effects / Q000502:physiology; D012849:Sinoatrial Node / Q000187:drug effects*
D011976:Receptors, Muscarinic; D002217:Carbachol