A pharmacologic study of calcium influx pathways in rabbit aortic smooth muscle

Date of Award




Degree Name

Doctor of Philosophy (Ph.D.)



First Committee Member

Cornelis van Breemen, Committee Chair


Functional characteristics and pharmacologic domains of receptor-operated and potential-sensitive calcium (Ca$\sp{2+}$) channels (ROCs and PSCs, resp.) were derived via measurements of $\sp{45}$Ca$\sp{2+}$ influx (M$\sp{\rm Ca}$) during activation by the neurotransmitters norepinephrine (NE), histamine (HS), and serotonin (5-HT) and by elevated extracellular potassium (K$\sp+$) in the individual or combined presence of organic Ca$\sp{2+}$ channel antagonists (CAts), calmodulin antagonists (Calm-ants), lanthanum (La$\sp{3+}$), and agents $\{$e.g., isoproterenol (Iso)$\}$ that increase intracellular levels of cyclic AMP. NE- and K$\sp+$-stimulated M$\sp{\rm Ca}$s through ROCs and PSCs, resp., were additive. NE-stimulated M$\sp{\rm Ca}$ was less sensitive than K$\sp+$-stimulated M$\sp{\rm Ca}$ to inhibition by the CAts D600, diltiazem (DZ), and nisoldipine (NS) and by Iso but was more sensitive to inhibition by forskolin, which elevated intracellular cyclic AMP 2.5-fold higher than Iso. NE- and K$\sp+$-stimulated M$\sp{\rm Ca}$s were identically sensitive to blockade by La$\sp{3+}$ and Calm-ants (phenothiazines, 48/80, and calmidozolium) and were equally dependent on extracellular (Ca$\sp{2+}$). M$\sp{\rm Ca}$-inhibitory potencies of Calm-ants correlated highly ($r$ =.97) with reported potencies of the same drugs for inhibition of calmodulin-dependent cellular processes; thus, a calmodulin-like moiety may constitute part of ROSc and PSCs. Measurements of aortic membrane potential showed that ROCs could be maximally activated at $-50$ mV and that PSCs were half-maximally activated at $-30$ mV. When these curves were fitted by computer to a two-site model, the PSC was less sensitive to CAt blockade as (NE) was increased, while ROC CAt sensitivity was unaffected by elevated (K$\sp+$). Thus, ROCs and PSCs behave as different channels with some mutual characteristics. Extracellular (CA$\sp{2+}$) competitively antagonized La$\sp{3+}$ blockade of NE- and K$\sp+$-stimulated M$\sp{\rm Ca}$s but noncompetitively reversed CAt (D600 and nisoldipine) and Calm-ant $\{$trifluoperazine (TFP)$\}$ inhibition of same, suggesting that Calm-ants and CAts interact with ROCs ad PSCs at a site(s) other than the "cation coordination site" to which La$\sp{3+}$ binds. DZ + Iso inhibition of the PSC was additive, and DZ inhibitions of total and Iso-resistant PSC-mediated M$\sp{\rm Ca}$ were identical. D600 reduced and DZ potentiated NS inhibition of the PSC. D600 and DZ enhanced but NS reduced TFP inhibition of the PSC. Using these and other results from the literature, a model of an aortic PSC is constructed and discussed. (Abstract shortened with permission of author.)


Health Sciences, Pharmacy

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