Increased PKA activity and its influence on isoprenaline‐stimulated L‐type Ca2+ channels in the heart from ovariectomized rats

1 We previously showed that oestrogen confers cardioprotection by downregulating the cardiac β1-adrenoceptor (β1-AR). The present study examined the effect of oestrogen on the post β1-AR signalling cascade, with particular emphasis on the activity of protein kinase A (PKA) and its influence on the L-type Ca2+ channel. 2 Three groups of adult female Sprague–Dawley rats were used: sham-operated controls, bilaterally ovariectomized (Ovx) rats, and Ovx rats with oestrogen replacement (Ovx+E2), which restored the oestrogen concentration to normal. 3 The electrically induced intracellular Ca2+ transient (E[Ca2+]i), 45Ca2+-uptake through cardiac L-type Ca2+ channels (Ca2+ channels), heart rate and force of contraction in response to β-AR stimulation with 10 nM isoprenaline (Iso) in hearts from Ovx rats were significantly greater than those of control and Ovx+E2 rats. The basal and Iso-induced PKA activities were also higher in hearts from Ovx rats. KT5720, a selective PKA inhibitor, completely inhibited its potentiating effect on basal Ca2+ channel activity in the Ovx rat heart. On the other hand, expression of G proteins (Gαs and Gαi1−3), basal and forskolin-stimulated cAMP accumulation, and responsiveness of PKA to cAMP, were not altered by Ovx. 4 Interestingly, the PKA inhibitor at the same concentration significantly reduced the increases in PKA activity and Ca2+ channel activity upon β-AR stimulation in all three groups of rats and the inhibitions were significantly greater in the Ovx rat than in the other two groups of rats. 5 This study provides the first evidence that, in addition to downregulation of β1-AR shown previously, suppression of PKA activity, which is partly responsible for the suppressed Ca2+ channel activity, also determines the E[Ca2+]i and cardiac contractility following β-AR stimulation in the female rat. British Journal of Pharmacology (2005) 144, 972–981. doi:10.1038/sj.bjp.0706123