Alpha(1)-adrenoceptor subtypes mediate negative inotropy in myocardium from alpha(1A/C)-knockout and wild type mice.

Cardiac alpha(1)-adrenoceptors (AR) have two predominant subtypes (alpha(1A)-AR and alpha(1B)-AR) however, their roles in regulating contraction are unclear. We determined the effects of stimulating alpha(1A)-AR (using the subtype-selective agonist A61603) and alpha(1B)-AR (using a gene knockout mouse lacking alpha(1A)-AR) separately, and together (using phenylephrine) on Ca(2+) transients, intracellular pH, and contraction of mouse cardiac trabeculae. Stimulation of alpha(1)-AR subtypes separately or together caused a triphasic contractile response. After a transient ( approximately 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 alpha(1A)-AR plus alpha(1B)-AR stimulation (50+/-3%) was more than with separate subtype stimulation (P<0.01), suggesting alpha(1A)-AR and alpha(1B)-AR mediate additive effects during phase 2. Force decline in phase 2 paralleled decreases of Ca(2+) transients that were reduced more with combined vs. separate subtype stimulation. During phase 3 the final force reduction was similar with stimulation of alpha(1A)-AR (20+/-5%), or alpha(1B)-AR (20+/-3%), or both (26+/-4%) suggesting alpha(1A)-AR and alpha(1B)-AR mediate non-additive effects during phase 3. In contrast, Ca(2+) transients recovered fully in phase 3 suggesting reduced force in phase 3 involved decreased myofilament Ca(2+)-sensitivity. Decreased Ca(2+)-sensitivity was not mediated by changes of intracellular pH since this was not affected by alpha(1)-AR stimulation. In contrast to mouse trabeculae, rat trabeculae demonstrated a positive inotropic response to alpha(1)-AR stimulation. In conclusion, for mouse myocardium in vitro both alpha(1)-adrenoceptor subtypes mediate negative inotropy involving decreased Ca(2+) transients and a decreased Ca(2+) sensitivity that does not involve altered intracellular pH.