A 47-year-old man comes to the office for follow-up of his elevated blood pressure. The patient's medical history is significant for stable angina pectoris, for which he takes sublingual nitroglycerin as needed. He has no orthopnea, paroxysmal nocturnal dyspnea, or lower extremity swelling. His blood pressure is 154/107 mm Hg and pulse is 86/min. Physical examination shows no abnormalities. The patient is started on a low dose of atenolol. Which of the following cellular changes will most likely occur as a direct effect of this medication?

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11ec160b_3b7f_916c_9249_972e94094ba2_MD0001_00 Beta blockers are the preferred treatment for patients with chronic stable angina and hypertension. They relieve anginal symptoms by inhibiting sympathetic stimulation of the heart, reducing both heart rate and contractility. Because beta blockers reduce heart rate and blood pressure during rest and exercise, the onset of angina is avoided or delayed. At low doses, atenolol is a selective β1 adrenergic antagonist. β₁ receptors are found in cardiac tissue and on renal juxtaglomerular cells, but not in vascular smooth muscle. The β₁ receptor is a G protein-coupled receptor (GPCR) associated with Gs, which increases intracellular cAMP levels. Blockade of the β₁ receptor leads to decreased cAMP levels in cardiac and renal tissue without significantly affecting cAMP levels in vascular smooth muscle (Choices C and D). GPCRs comprise a large family of 7 transmembrane domain receptors that activate intracellular second messenger systems. There are 2 principal second messenger systems involving GPCRs: (Choice A) Propranolol is a nonselective beta blocker that blocks both β1 and β2 adrenergic receptors. β2 receptors are found in vascular and bronchial smooth muscle. Blockade of β1and β2 receptors would therefore result in decreased cAMP levels in cardiac myocytes, renal juxtaglomerular cells, and vascular smooth muscle. (Choice E) A medication such as diltiazem, a non-dihydropyridine calcium channel blocker, would not directly affect cAMP levels. Educational objective: β₁ adrenergic receptors are found in cardiac tissue and on renal juxtaglomerular cells, but not in vascular smooth muscle. Selective blockade of the β₁ receptor (eg, with atenolol) leads to decreased cAMP levels in cardiac and renal tissue without significantly affecting cAMP levels in vascular smooth muscle. __________ References: ß-adrenergic receptor signaling in cardiac function and heart failure. (http://www.ncbi.nlm.nih.gov/pubmed/18523538)

A 47-Year-Old Man Comes to the Office for Follow-Up of His