<p><strong>Telmisartan</strong>: Angiotensin II is formed from angiotensin I in a reaction catalyzed by angiotensin-converting enzyme (ACE, kininase II). Angiotensin II is the principal&nbsp;pressor&nbsp;agent of the renin-angiotensin system, with effects that include&nbsp;vasoconstriction, stimulation of synthesis and release of&nbsp;aldosterone, cardiac stimulation, and renal reabsorption of sodium. Telmisartan blocks the vasoconstrictor and aldosteronesecreting effects of angiotensin II by selectively blocking the binding of angiotensin II to the AT1 receptor in many tissues, such as&nbsp;vascular&nbsp;smooth muscle&nbsp;and the&nbsp;adrenal gland. Its action is therefore independent of the pathways for angiotensin II synthesis.<br />
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There is also an AT2 receptor found in many tissues, but AT2 is not known to be associated with cardiovascular homeostasis. Telmisartan has much greater affinity (&gt;3,000-fold) for the AT1 receptor than for the AT2 receptor.<br />
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Telmisartan does not inhibit ACE (kininase II) nor does it bind to or block other hormone receptors or ion channels known to be important in cardiovascular regulation.<br />
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Blockade of the angiotensin II receptor inhibits the negative regulatory feedback of angiotensin II on renin secretion, but the resulting increased plasma renin activity and angiotensin II circulating levels do not overcome the effect of telmisartan on blood pressure.<br />
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<strong>Hydrochlorothiazide</strong>: Hydrochlorothiazide is a thiazide diuretic. Thiazides affect the renal tubular mechanisms of&nbsp;electrolyte&nbsp;reabsorption, directly increasing excretion of sodium salt and chloride in approximately equivalent amounts. Indirectly, the diuretic action of hydrochlorothiazide reduces plasma volume, with consequent increases in plasma renin activity, increases in aldosterone secretion, increases in urinary&nbsp;potassium&nbsp;loss, and decreases in serum potassium. The renin-aldosterone link is mediated by angiotensin II, so coadministration of an ARB tends to reverse the potassium loss associated with these&nbsp;diuretics. The mechanism of the antihypertensive effect of thiazides is not fully understood.</p>