Electrophysiologists conduct a study in which they record the membrane potential changes of cardiac cells while exposing them to various agents. A tracing from one of the experiments is shown below. Which of the following substances was most likely applied to the cells at the point indicated by the arrow?

Question

Electrophysiologists conduct a study in which they record the membrane potential changes of cardiac cells while exposing them to various agents.  A tracing from one of the experiments is shown below.  Which of the following substances was most likely applied to the cells at the point indicated by the arrow?

Quizplus · Accepted Answer

11ec160b_3c8d_30b2_9249_1b08b7b249e4_MD0001_00 The membrane potential changes shown above are typical of cardiac pacemaker cells, which are primarily located in the sinoatrial node on the wall of the right atrium. These specialized cells demonstrate slow spontaneous depolarization at regular intervals (automaticity) and have an action potential that differs from that of cardiomyocytes. The action potential of pacemaker cells proceeds as follows: Phase 0 (upstroke) begins once the cell crosses the depolarization threshold (about -40 mV). It is characterized by the opening of voltage-gated L-type (long-lasting) Ca²⁺ channels with a large influx of Ca²⁺ into the cell. Phase 3 (repolarization) begins with closure of L-type Ca²⁺ channels in conjunction with the opening of K⁺ channels and the efflux of K⁺ ions from the cell. Phase 4 (pacemaker potential) is characterized by the slow influx of Na⁺ that begins at the end of repolarization. During this time, there is also a slow decrease in K⁺ efflux as the K⁺ channels continue to close. Once the membrane potential reaches approximately -50 mV, the T-type (transient) Ca²⁺ channels open and allow Ca²⁺ to enter the cell and contribute to depolarization. As the action potential approaches -40 mV, the L-type (long-lasting) Ca²⁺ channels open and trigger an action potential. Adenosine and acetylcholine affect phase 4 of the action potential, reducing the rate of spontaneous depolarization in cardiac pacemaker cells. Adenosine interacts with A1 receptors on the surface of cardiac cells, activating potassium channels and increasing potassium conductance, causing the membrane potential to remain negative for a longer period. Adenosine also inhibits L-type Ca²⁺ channels, further prolonging the depolarization time. These actions result in a transient slowing of the sinus rate and an increase in atrioventricular (AV) nodal conduction delay. Adenosine is useful in the termination of paroxysmal supraventricular tachycardia. Acetylcholine behaves similarly by increasing outward K⁺ conductance while decreasing inward Ca²⁺ and Na⁺ currents during phase 4. (Choices B and C) Angiotensin II is a potent vasoconstrictor that increases both systolic and diastolic blood pressure. Aldosterone acts on collecting duct epithelium in the kidneys; this increases Na⁺ reabsorption and urinary excretion of K⁺ and H⁺. However, neither process directly affects phase 4 of pacemaker action potential. (Choices D and E) Glycine is an inhibitory neurotransmitter, and glucagon is a pancreatic hormone that increases blood sugar levels. Neither affects phase 4 of pacemaker action potential. (Choice F) Norepinephrine facilitates opening of L-type Ca²⁺ channels and Na⁺ channels in phase 4 of the nodal action potential. This leads to more rapid depolarization. Educational objective: The phase 4 slow depolarization in cardiac pacemaker cells occurs due to the closure of repolarizing K⁺ channels, the slow influx of Na⁺ through funny channels, and the opening of T-type Ca²⁺ channels. Acetylcholine and adenosine reduce the rate of spontaneous depolarization in cardiac pacemaker cells by prolonging phase 4. __________ References: Sophisticated architecture is required for the sinoatrial node to perform its normal pacemaker function. (http://www.ncbi.nlm.nih.gov/pubmed/12625620)

Electrophysiologists Conduct a Study in Which They Record the Membrane