D. T. McCloskey, D. G. Rokosh, T. D. O'Connell, E. C. Keung, P. C. Simpson and A. J. Baker. α₁-Adrenoceptor Subtypes Mediate Negative Inotropy in Myocardium from α_1A/C-Knockout and Wild Type Mice. Journal of Molecular and Cellular Cardiology (2002) 34, 1007–1017. Cardiac α₁-adrenoceptors (AR) have two predominant subtypes (α_1A-AR and α_1B-AR) however, their roles in regulating contraction are unclear. We determined the effects of stimulating α_1A-AR (using the subtype-selective agonist A61603) and α_1B-AR (using a gene knockout mouse lacking α_1A-AR) separately, and together (using phenylephrine) on Ca²⁺ transients, intracellular pH, and contraction of mouse cardiac trabeculae. Stimulation of α₁-AR subtypes separately or together caused a triphasic contractile response. After a transient (≈3%) force rise (phase 1), force declined markedly (phase 2), then partially recovered (phase 3). In phase 2, the force decline (% of initial) with combined α_1A-AR plus α_1B-AR stimulation (50±3%) was more than with separate subtype stimulation (P<0.01), suggesting α_1A-AR and α_1B-AR mediate additive effects during phase 2. Force decline in phase 2 paralleled decreases of Ca²⁺ transients that were reduced more with combined vs. separate subtype stimulation. During phase 3 the final force reduction was similar with stimulation of α_1A-AR (20±5%), or α_1B-AR (20±3%), or both (26±4%) suggesting α_1A-AR and α_1B-AR mediate non-additive effects during phase 3. In contrast, Ca²⁺ transients recovered fully in phase 3 suggesting reduced force in phase 3 involved decreased myofilament Ca²⁺-sensitivity. Decreased Ca²⁺-sensitivity was not mediated by changes of intracellular pH since this was not affected by α₁-AR stimulation. In contrast to mouse trabeculae, rat trabeculae demonstrated a positive inotropic response to α₁-AR stimulation. In conclusion, for mouse myocardium in vitro both α₁-adrenoceptor subtypes mediate negative inotropy involving decreased Ca²⁺ transients and a decreased Ca²⁺ sensitivity that does not involve altered intracellular pH.