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 (-5[1])expulsion of this ion to (-5[1])K+ intake (-5[2])is blocked by omeprazole. This ion's (-5[1])secretion alongside intrinsic factor by (*) parietal cells is blocked by PPIs. High concentration of this ion turns pepsinogen into its active form, pepsin. Because chyme is rich in this ion, bicarbonate is released to neutralize it upon entry into the small intestine. For 10 points, stomach acid is rich in what cation? (10[1])■END■ (10[1]0[2])

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