Changes in Cytoplasmic Free Ca 2 + in Response to Norepinephrine , Angiotensin II

Addition of norepinephrine, angiotensin II, or histamine leads to a transient rise in the cytoplasmic (i' concentration (ICa2 I), as measured with aequorin, in rabbit aortic strips. Each induces a 1Ca2ill transient which peaks in 2 min and then falls either back to baseline (angiotensin II) or to a plateau (norepinephrine aid histamine). The ICa2 transient is due to the mobilization of Ca2+ from a caffine-sensitive, intracellular pool. An elevation of 11+] to 35 mM leads to a monotonic sustained rise in ICa2l1l which depends entirely on extracellular Ca2+, but an increase to 100 mM leads to a [Ca2+Jj transient from the mobilization of intracellular Ca2+. Atrial natriuretic peptide does not alter basal [Ca2+h nor inhibit the ICa2?+i transient induced by either histamine or angiotensin II, but blocks that induced by norepinephrine, and blocks the plateau phase induced by either histamine or norepinbphrine. The peptide inhibits the contractile response to all three agonists and to KV.