The intrinsic heart rate of most vertebrates studied, including humans, is elevated during digestion, suggesting that a nonadrenergic-noncholinergic factor contributes to the postprandial tachycardia. The regulating factor, however, remains elusive and difficult to identify. Pythons can ingest very large meals, and digestion is associated with a marked rise in metabolism that is sustained for several days. The metabolic rise causes more than a doubling of heart rate and a fourfold rise in cardiac output. This makes the python an interesting model to investigate the postprandial tachycardia. We measured blood pressure and heart rate in fasting Python regius, and at 24 and 48 h after ingestion of a meal amounting to 25% of body wt. Digestion caused heart rate to increase from 25 to 56 min, whereas blood pressure was unchanged. The postprandial rise in heart rate was partially due to a doubling of intrinsic heart rate. The H(2)-antagonist did not affect heart rate of fasting snakes but decreased heart rate by 15-20 min at 24 h into digestion, whereas it had no effects at 48 h. Thus, the histaminergic tone on the heart rose from none to 30% at 24 h but vanished after 48 h. In anesthetized snakes, histamine caused a systemic vasodilatation and a marked increase in heart rate and cardiac output mediated through a direct effect on H(2)- receptors. Our study strongly indicates that histamine regulates heart rate during the initial phase of digestion in pythons. This study describes a novel regulation of the vertebrate heart.

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The voltage‐gated L‐type Ca channels (LTCCs) are important in smooth muscle excitation‐transcription coupling. We have developed a method for downregulation of the LTCC in rat small mesenteric arteries in vivo by the use of siRNA. Transfection with specific siRNA decreased LTCC expression by ~75%. The downregulated arteries developed reduced wall stress in response to K‐induced depolarization which resembled control arteriess pre‐treated with nifedipine. Changes in wall stress correlated with smaller changes in [Ca2+]i in the arteries downregulated for LTCC. Similar changes were seen with noradrenalin and vasopressin.In contrast to the effect of LTCC blocker chronic treatment, the LTCC downregulated arteries had increase in media thickness in comparison to the controls. The remodeling was associated with phenotypic changes of the VSMCs: contractile phenotype markers (myocardin, tropomyosin 4, smooth muscle myosin heavy chain, smooth muscle α‐actin and h‐caldesmon) were significantly downregulated, while marker genes for the proliferative phenotype (l‐caldesmon and ki‐67) were significantly upregulated. In α‐toxin permeabilized arteries downregulated for LTCC the force development to calcium was reduced compared to control arteries. We conclude that LTCC expression and/or activity are crucial in determining VSMC phenotype as well as the structure of small resistance arteries.