Round 2: Tossup 18

Expulsion of this ion is coupled to intake of another by antiporters in the PCT that are triggered by angiotensin II. Enterochromaffin-like cells produce histamine that triggers the release of this cation by binding to H2 receptors. An ATPase that couples expulsion of this ion to K+ (-5[1])intake is blocked by omeprazole. This ion's secretion alongside intrinsic factor by (*) parietal cells is blocked by PPIs. (-5[1])High (10[1])concentration of this ion turns (-5[1])pepsinogen into its active form, pepsin. Because chyme is rich in this ion, bicarbonate (-5[1])is released to neutralize it upon entry into the small intestine. For 10 points, (-5[1])stomach acid is rich in what cation? ■END■ (0[3])

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